Oligo- and Polymerization of Olefins

6.06

Oligo- and Polymerization of Olefins

R Ko¨hn, University of Bath, Bath, UK ã 2013 Elsevier Ltd. All rights reserved.

6.06.1 6.06.2 6.06.2.1 6.06.2.1.1 6.06.2.1.2 6.06.2.1.3 6.06.2.1.4 6.06.2.2 6.06.2.2.1 6.06.2.2.2 6.06.2.2.3 6.06.2.3 6.06.2.4 6.06.2.5 6.06.2.5.1 6.06.2.5.2 6.06.3 6.06.3.1 6.06.3.1.1 6.06.3.1.2 6.06.3.2 6.06.3.2.1 6.06.3.2.2 6.06.3.3 6.06.3.4 6.06.3.5 6.06.3.5.1 6.06.3.5.2 6.06.3.5.3 6.06.3.5.4 6.06.4 6.06.4.1 6.06.4.2 6.06.4.3 6.06.4.3.1 6.06.4.3.2 6.06.4.3.3 6.06.4.4 6.06.4.5 6.06.4.6 6.06.4.7 6.06.4.8 6.06.4.9 6.06.4.10 6.06.5 6.06.5.1 6.06.5.2 6.06.5.3 6.06.5.4 6.06.5.5 6.06.5.6 6.06.5.7 6.06.5.8 6.06.6

Introduction Polymerization Mechanisms Coordination Polymerization General mechanistic overview Initiation Propagation mechanisms Chain termination steps Olefin Metathesis (Carbene) Mechanisms Ring Ring-opening metathesis polymerization Acyclic diene metahesis polymerization Green–Rooney mechanism Cationic Polymerization Anionic Polymerization Radical Polymerization Organometallic radical polymerization Atom transfer radical polymerization Control Over Polymer Structure Linear Oligo- and Polymers High-density polyethylene Stereo and tacticity control of a-olefin polymers Branched Polymers and Copolymers Branched polymers from homo-polymerization Branched polymers from copolymerization Block-copolymerization Cyclic Polymers Control of Molecular Weight Oligomerization Dimerization Selective trimerization Selective tetramerization Catalysts by Metal Center Main Group Metals (Including Group 12) Group 3 (Including Lanthanides) Group 4 Metallocenes Half-metallocenes Non-metallocenes Group 5 Group 6 Group 7 Group 8 Group 9 Group 10 Group 11 Co-Catalysts, Activators, and Promoters Methylalumoxane Other Aluminum Alkyl Compounds Boranes Brønsted Acids with Weakly Coordinating Anions Trityl Salts of Weakly Coordinating Anions Oxidizing Salts of Weakly Coordinating Anions Oxidizing Additives (Halide Effects) Nucleating Additives Monomers for Olefin Polymerization

Comprehensive Inorganic Chemistry II

http://dx.doi.org/10.1016/B978-0-08-097774-4.00607-0

128 128 128 128 129 129 133 134 134 134 135 136 136 137 137 138 138 138 138 138 140 140 140 140 141 141 142 142 142 142 143 143 143 143 143 143 144 144 145 146 146 147 147 147 148 148 148 148 149 149 149 149 149 149

127

128

Oligo- and Polymerization of Olefins

6.06.6.1 6.06.6.2 6.06.6.3 6.06.6.4 6.06.6.5 6.06.6.6 6.06.6.7 6.06.6.8 6.06.7 6.06.7.1 6.06.7.2 6.06.8 References

6.06.1

Ethylene a-Olefins Cyclic Olefins and Non-conjugated Dienes Higher Substituted Olefins Styrene Butadienes CO (for copolymers) Other Functionalized Monomers Heterogeneous Polymerization Catalysts ZN Catalysts Phillips Catalysts Conclusion

Introduction

Sixty years have passed since the discovery of the first catalysts for the polymerization of alkenes, and the impact on our society – which we cannot envision without synthetic polymers – has been enormous. Nowadays, metal-catalyzed polymerization represents the most powerful, benign, and sustainable method for the controlled synthesis of polymeric materials. Today, most metals are involved in the catalysis of olefins via several very different mechanisms, leading to a broad range of polymer products produced at well more than 100 million tons per year. The history of polyolefins began about 100 years ago with the discovery of methods to polymerize olefins without intimate metal involvement, for example, by anionic or radical pathways. Some of these processes are still used today, but they offer limited control over the polymerization. The 1950s saw the introduction of heterogeneous metal catalysts for the controlled olefin polymerization, leading to widespread applications throughout our society. The “metallocene revolution” starting in the 1970s led to highly active homogeneous catalysts that allowed rational ligand design to produce specific polymer structures. By the 1990s, new non- or post-metallocenes were introduced, which were easier to assemble and produced even higher activity and control over polymer structure.1 This chapter provides an overview of metal-assisted polymerization of monomers that contain C–C double bonds (“olefins”) and utilize the double bonds to connect the monomers. Metathesis polymerization (see Section 6.06.2.2) does not strictly follow this definition because the C–C double bond is broken and either one or both “C1”-fragments are used to build the polymer. Other forms of C1 polymerizations, which could be considered as polymerization of “half-olefin” or carbene units, have been reviewed recently.2 The literature about catalytic olefin polymerization is extensive, with about 1000 references per year over the last decade (SciFinder search for olefin polymerization) and hundreds of review articles covering parts of the subject. Therefore, this chapter mainly references recent key review articles published before October 2011 rather than the primary literature. The focus of this overview is on the different mechanistic pathways to polymerization of olefins, including functionalized olefins and the role different metals play, rather than a comprehensive list of ligands and catalyst activities. The latter is covered by comprehensive reviews. Thus, some important contributions are referenced only indirectly via the reviews.

149 150 150 150 151 151 152 152 152 152 152 153 153

6.06.2

Polymerization Mechanisms

The polymerization of olefins can be catalyzed by many different metal compounds and by a variety of very different mechanisms. The major mechanisms involving metals will be covered in the following sections. Each mechanism is particularly suited for certain types of olefins. The simplest olefins (ethylene and terminal “a-olefins”) are most commonly polymerized by “coordination polymerization” and, as the “standard” mechanism, this mechanism will be discussed first. Other mechanisms may have some common features with coordination polymerization and often use similar metal precatalysts. The other mechanisms often are more suitable to other types of olefins, for example, highly substituted olefins (cationic polymerization), strained cyclic olefins (ring opening metathesis polymerization, ROMP) or functionalized olefins and styrenes (radical polymerization). Metals may not be as intimately involved in the other mechanisms as in coordination polymerization. In some cases, different mechanistic pathways are possible for very similar catalysts and substrates.

6.06.2.1

Coordination Polymerization

6.06.2.1.1 General mechanistic overview The standard mechanism for coordination polymerization catalyzed by transition metals is the Cossee–Arlman type mechanism, shown in Scheme 1. Other mechanisms and variations will be discussed later. The initial polymerization active complex is a metal–alkyl with a free coordination site to bind the olefin. This complex is often cationic to increase the Lewis acidity. A cation may also reduce the propensity to dimerize, as seen in some analogous neutral complexes.3 This “active” form is usually reached in initiation steps from a precatalyst by the addition of activators. The relative rate of propagation by olefin insertion into the metal–alkyl bond to chain transfer, usually via b-hydrogen elimination, strongly depends on the position of the metal in the periodic table. Early transition metals tend to have relatively fast olefin insertion, and highmolecular-weight polymerization catalysts initially were found for these metals, for example, Ziegler–Natta (ZN) catalysts. Late transition metals tend to have stronger olefin-p-coordination, favoring b-hydrogen elimination, and low-molecular-weight oligomerization, rather than polymerization, initially was observed for late transition metals, for example, nickel-based SHOP catalysts (see Section 6.06.3.5.1). However, the discovery

Oligo- and Polymerization of Olefins

R

R M

M Insertion

+

+n

(R = Et) M

R n

+

R M - Polymer

M

H

- Polymer M

H

M β-H elimination

R n M β-H transfer

R n+1

R n+1

+

Scheme 1 Typical Cossee–Arlman type mechanism.

by Ittel and co-workers4 that steric bulk can slow down chain transfer despite fast b-hydrogen elimination has led to a surge of new late transition metal polymerization catalysts.

6.06.2.1.2 Initiation 5,6 6.06.2.1.2.1 Alkylation Coordination polymerization requires a metal–carbon bond for the insertion of the olefin monomers. In many cases, organometallic complexes that contain a suitable metal–alkyl function for the subsequent activation can be synthesized. However, early transition metal organometallic compounds are particularly sensitive to air and moisture, and the preferred precatalysts are more stable complexes that require activation by alkylation. By far the most common alkylating co-catalysts are aluminum alkyl compounds. They are generally highly soluble in nonpolar solvents and do not generate insoluble by products after alkylation as organolithium or magnesium compounds. Common reagents are AlMe3, AlEt3, Et2AlCl, AliBu3, or methylalumoxane (MAO). The reagents vary in their Lewis acidity and reducing power, and a range of reagents usually has to be tested for optimal catalysis.

6.06.2.1.2.2 Generation of “vacant” coordination sites A second requirement for the coordination polymerization (and many other mechanisms listed later) is the generation of vacant sites for the coordination of the olefin monomer. If the complex cannot generate this site by dissociation of a weakly coordinated ligand, solvent molecule, or anion (see Section 6.06.2.1.2.3), a co-catalyst is required to remove a ligand. The co-catalysts usually use one of the following strategies: – Lewis acids can abstract anionic ligands in the metal coordination sphere. The most typical ligands for abstraction are halides or alkyl groups (Me is best) when more than one alkyl group is present. – An alkyl (or other protolyzable ligand) can be removed by protolysis with a Brønsted acid. The alkane produced is usually inert and does not affect the catalysis. – An alkyl (or other oxidizable ligand) can be removed by oxidation. Typical oxidants are silver(I) or ferrocenium salts of weakly coordinating anions. The alkyl radicals generated usually react with the medium/solvent and do not further affect the catalysis.

129

6.06.2.1.2.3 Single component catalysts The ultimate aim, from an academic point of view, is the synthesis of single component catalysts, that is, a complex that does not require any additional co-catalyst and will lead to polymerization as soon as a monomer is added. For coordination polymerization catalysts, this usually requires separate alkylation and anion abstraction steps with the isolation of the alkyl cation that contains a free coordination site (or at least a site that easily exchanges with olefin monomers). Wellcharacterized and active complexes of this type are rare and usually require very pure solvents and olefin monomers for polymerization, in the absence of other co-catalysts as “scavengers.”

6.06.2.1.3 Propagation mechanisms7 6.06.2.1.3.1 Cationic metal–alkyl complexes, the role of agostic interactions A good basic mechanism for coordination polymerization has been proposed by Cossee,8 as shown in Scheme 1. The important intermediate alkyl–olefin complex has never been observed for early transition metals with ethylene or a-olefins because of the low barrier to olefin insertion, but numerous calculations confirm its existence as an intermediate. Kinetic isotope studies have indicated that the olefin coordination is fast and reversible relative to insertion for early transition metal catalysts.9 However, the reversible coordination of olefin-containing tethers can be studied by NMR10–13 and binding energies of around 50 kJ mol
1 were found, lower for nonansa and higher for doubly bridged metallocenes. The structure of a 2,2-disubstituted olefin complex, for which insertion is slow, recently has been reported.14 Model complexes with noninserting OtBu or C6F5 instead of alkyl groups give NMR-detectable olefin complexes without the tether, and equilibria can be studied for different olefins.15,16 The next step is the insertion of the olefin into the metal– carbon bond. The insertion (and the elimination as its reverse) has been probed by Hammett studies. The transition state involves a partial charge build-up on the b-carbon of the olefin. Stabilization of the cationic alkyl complex by a b-agostic hydrogen interaction slows down the insertion, leading to a dependence of the insertion rate on the type of alkyl as M–Me > M–nPr > M–Et.17 The transition state of the olefin insertion into the metal– carbon bond requires an overlap of metal orbitals, the olefin p-orbitals and the alkyl s-orbital. Metal d-orbitals are particularly suitable for this interaction, and transition metals show a much higher insertion rate than main group metals. The alkyl s-orbital is usually very directional and overlaps little with the olefin p-orbitals. Highly Lewis acidic metal centers may be able to assist in the transition state by an a-agostic interaction, leading to a turning of the s-orbital toward the olefin, as shown in Scheme 2. This improvement on the Cossee–Arlman mechanism has been proposed as an extension of the Green–Rooney mechanism (see Section 6.06.2.2.3) to d0-complexes (Brookhart–Green mechanism).18 An important consequence of the proposed a-agostic interaction is that the polymer chain is placed outside the M–Calkyl–olefin plane and can adopt a preferential position either above or below the plane, depending on steric interactions with ancilliary ligands, the olefin, or both.

130

Oligo- and Polymerization of Olefins

Without any agostic interaction, the orientation of the polymer chain is less well-defined and may even lie in the plane with the b-agostic interaction usually found in the resting state. The proposed a-agostic mechanism has been supported by calculations19 and experimental deuterium isotope studies,20,21 and it has been reviewed previously.22,23 The inclusion of the polymer orientation via the a-agostic interaction has led to a modification of the understanding of the stereoselectivity for propylene polymerization in metallocene catalysts (see Section 6.06.3.1.2). Rather than direct enantiomorphic site control (control of propylene orientation by the ancillary ligand), the ancillary ligand controls the orientation of the polymer chain in the transition state of the insertion, and the propylene subsequently selects the orientation trans to the polymer chain, as shown in Scheme 3. The olefin coordination must occur in a reversible pre-equilibrium so that this relay mechanism in the transition state of the olefin insertion can control the stereoselectivity.24 This “indirect” enantiomorphic site control also explains the observed lack of selectivity for propylene orientation for insertion into a metal–methyl bond (P¼H in Scheme 3). The g-agostic interaction resulting from the insertion into the a-agostic complex may also inhibit inversion at the metal (chain swinging or epimerization). A strictly alternating olefin insertion (i.e., no chain swinging) is a requirement for high syndiospecificity (see Section 6.06.3.1.2). Computational studies of neutral salicylaldminato nickel catalysts suggest that a similar a-agostic interaction in the transition state is also important for late transition metals.25 Generally, b-agostic complexes are thermodynamically the most stable form of metal–alkyl complexes with an “empty” coordination site. These agostic structures are particularly important for late transition metals and may lead to a preference of olefin hydrides for third-row transition metals. The importance of b-agostic interactions to the bonding of alkyl groups to unsaturated metals recently has been explored computationally for cationic metallocenes and Brookhart’s nickel and palladium complexes, as well as neutral palladium complexes.26 The b-agostic interaction was found to be rather constant across all equally charged complexes and significantly higher for cationic complexes than for neutral ones, contributing more than 10% to the total binding energy of the alkyl groups. Thus, late transition metals in particular never form a true 16-electron species,

HH

H

P M

and coordination of incoming olefin monomers can best be regarded as a substitution of the agostic interaction. Thus, agostic interactions play an important role in the stereoselectivity and stabilization of intermediates during olefin polymerization. 6.06.2.1.3.2 Role of the anion 27 Most polymerization catalysts are cationic complexes in the active form. The positive charge increases the Lewis acidity of the complex, while reducing the fraction of any dinuclear alkyl bridged complexes. With low dielectric constants, the solvents typically used are unlikely to separate the cations completely from their anions, even in the case of the most weakly coordinating anions, due to the electrostatic attraction. Thus, ion pairs will be present either as contact- or solvent-separated ion pairs if the anion is not part of the metal’s coordination sphere (inner-sphere ion pair). Yet, the role of the anion has initially been largely ignored. However, the stability and catalytic performance is often highly dependent on the anion employed, and significant efforts have been put into the syntheses and studies of ever more weakly coordinating anions. The effect of the co-catalyst on the polymerization activity was reviewed by Chen and Marks in 2000.28 Anion effects are most pronounced when low permittivity solvents are used (e.g., toluene typically used for early transition metal catalysts) and become less important for functional group-tolerant late transition metal catalysts used in more polar solvents, for example, dichloromethane. Structural information about the ion pair in solution is difficult to obtain. In some cases, long-range distance-dependent NOESY methods have been able to locate the anion in the anion pair, in particular by using 19F atoms in the anion, and DOSY experiments give information on the size of the diffusing molecule (cation, ion pair, or larger aggregates).27 The exact mechanism of anion displacement from inner sphere ion pairs is still unclear and could be either associative or dissociative. Even excess co-catalyst (and thus anion) can have an effect on catalytic activity in some cases. An interesting recent hypothesis for the role of the anion is the entropy effect of anion displacement by the monomer.29 Catalysts with an inner sphere contact ion pair will react with the monomer to give an outer sphere contact ion pair and the monomer coordinated to the metal where the anion was

H H

P

M

H

H

P H

M

M

H P

β-Agostic resting state

γ-Agostic

α-Agostic

H

Scheme 2 Role of agostic hydrogens in olefin insertions.

Ln M+ P

Ln M+ P

Ln M+

H H

P

H H

Ln M+

Ln M+ H

H

Slow

P P

Scheme 3 Stereo chemistry in olefin insertions.

Ln M+

Chain swinging

P

131

Oligo- and Polymerization of Olefins

attached. The degrees of freedom lost in the monomer results in a large entropy penalty, which can be on the order of
100 J K
1 mol
1. However, if the resting state of the catalyst is an outer sphere ion pair with a solvent molecule occupying the vacant site, exchange of this solvent molecule with the monomer is nearly entropy-neutral, resulting in accelerated polymerization. This effect may account for several orders of magnitude in activity. However, accurate thermodynamic data for the propagation rate in systems of interest are rare. Another effect of the anion on the catalysis can be mediated by weak interactions with the polymer chain. C–F. . .H–C interactions between the anion (or other C–F containing groups within the ligand) can lead to specific orientations of the coordinated polymer chain, resulting in some influence on the polymerization performance.30 Thus, catalyst performance generally improves as the coordinating anion becomes weaker. A selection of weakly coordinating anions explored in polymerization is shown in Figure 1, with more details in Chapters 1.22–1.24. An alternative approach to catalyst design with minimal cation–anion interaction is to reduce the coordinating ability of the cation. Steric bulk on the ligands can reduce the interaction with the anion. Steric bulk can also slow down the monomer coordination. However, the size of the anion is usually very different from the monomer and positioning steric bulk far enough from the metal center can keep the anion from coordinating, while still allowing easy access for the small monomer.31 6.06.2.1.3.3 Neutral metal–alkyl complexes While cationic catalysts have been very successful, they also have their disadvantages. Cationic complexes are electrophilic and sensitive to protic solvents. Neutral complexes are often more tolerant to solvents with donor groups and may offer better solubility than ionic complexes, without a need for weakly coordinating anions that are often expensive. Neutral nickel complexes with anionic ligands have been very successful for ethylene oligomerization since the 1970s (SHOP catalysts in Section 6.06.3.5.1). Shortly after the discovery of the polymerization enhancing effect of bulky axial substituents in cationic a-diimine nickel complexes, analogues of the neutral SHOP catalysts were modified to incorporate axial bulk. Salicylaldimine and anilinotroponone ligands, shown in Scheme 4, gave reasonable polymerization activity. Activities for the SHOP, as well as analogous polymerization catalysts, were found to decrease as the chelate ring size was increased from the five-membered ring. While polar solvent additives decrease the activity, polymerization was observed even in the presence of

F

F3C

F

F

F3C F3C

B C N B F

F F

Figure 1 Selection of weakly coordinating anions.

O O Al O O

CF3 F3C F3C

CF3

THF, water, or NEt3.32 The complexes suffer from relatively short lifetimes, due to deactivation to bis-ligated complexes. This decomposition pathway can be blocked by the introduction of further steric bulk directed toward the metal center, resulting in a longer lifetime and higher activity.33 6.06.2.1.3.4 Metallacyclic mechanisms including role in selective tri- and tetramerization34 One variation of the coordination polymerization mechanism is a mechanism involving metallacycles. One of the earliest transition metal-based ethylene polymerization catalysts, the Phillips catalyst, based on CrO3 on silica (see Section 6.06.7.2), can yield active catalysts even in the absence of any alkylating reagent. This raised the question of how metal carbon bonds were formed to allow ethylene insertion. A number of mechanisms have been proposed,35 among them a mechanism via oxidative addition of two ethylene molecules to a reduced chromium species to form chromacycles, in which additional ethylene can insert into the Cr–C bonds following the general mechanism for coordination polymerization. A metallacycle formation from reduced chromium and two ethylene molecules would explain the activity of the Phillips catalyst without any organometallic activator. In addition, it was noted that some ethylene trimer, 1-hexene, can be produced selectively over other oligomers under certain circumstances and subsequently copolymerized to give butyl side chains as a major branch type in the polyethylene produced by Phillips catalysts. Homogeneous chromium complexes were subsequently found that yield this unusual selective trimerization almost exclusively,36,37 therefore allowing a detailed mechanistic

Ph3P

Ph3P

Ph Ni N

O

Ar

O

Ph Ni N

py

Me Ni

Ar O

N

R

Decomposition R

No bis-ligated complex

Ar N

O Ni

O

N Ar

R

Scheme 4 Neutral nickel complexes, py ¼ pyridine.

CF3 CF3 CF3 CF3 CF3

B(C6F5)3 N C (C6F5)3B N C Ni C N B(C6F5)3 C N B(C6F5)3

Ar

132

Oligo- and Polymerization of Olefins

investigation of the trimerization process (see also Section 6.06.3.5.3). The greater than 99% selectivity for the trimer found in some of these systems excluded the possibility of a regular insertion mechanism, which would result in a Flory– Schulz-like distribution of oligomers. One attractive alternative is a route via metallacycles, as shown in Scheme 5. Over a decade of ligand optimization has yielded catalyst with high activity and selectivity for 1-hexene. Some examples are shown in Section 6.06.4.5. Selective trimerization has also been extended to a-olefins and styrene with pre-catalyst 3, although at a much lower rate.38 Metallacycles have been known and well-studied for many decades. They show a much higher stability of the intermediate metallacyclopentane relative to the ring-expanded metallacycloheptane. The latter has the flexibility to allow b-hydrogen elimination, followed by reductive elimination of the metal– alkyl hydride or direct b-hydrogen transfer to the other alkyl group. This was supported experimentally for donor stabilized chromacycles by Jolly.39 A strong evidence for metallacycles in the selective trimerization was found by isotope studies using a mix of C2H4 and C2D4, resulting only in oligomers containing multiples of four D atoms.40 A regular insertion mechanism, somehow stopping at three units, would transfer one H/D atom to the next trimer, resulting in oligomers also containing (4n  1) D atoms. The mechanism requires a change of oxidation state by two. While the involvement of all oxidation states from Cr(I) to Cr (V) have been proposed, experimental and computational evidence seems to point to a Cr(I/III) cycle with some evidence also for Cr(II/IV). The isolation of bis(arene)chromium(I) complexes as decomposition product during the trimerization proves that chromium(I) is accessible under the catalytic conditions.41 Computational studies have supported both alternatives, although most of these studies suffered from the uncertainty of the spin state involved. A most recent study allowed spin crossing during the calculated cycle and found the lowest energy pathway for a Cr(I/III) cycle with spin crossover from S ¼ 4 to 6 in the reduced species.42 Similar selective trimerization catalysis was subsequently found for other metals as well, for example, Ti(II/IV) (see M(n)

2

m+1

M(n+2)

M(n+2) m+1 H M(n+2) M(n+2)

m

m+1 Scheme 5

Metallacyclic mechanism of selective olefin oligomerization.

CrLn

CrLn H

Scheme 6 Formation of methylcyclopentane via hydride elimination.

Section 6.06.4.3.2) and Ta(III/V) (see Section 6.06.4.4). Both systems are diamagnetic and detailed DFT studies on the Ti system confirmed the viability of the metallacyclic mechanism. The precise mechanism for the reductive elimination of the trimer has also been controversial. The originally proposed mechanism via metal–alkyl hydride species was found to be unfavorable relative to direct hydride transfer from one alkyl arm of the metallacycle to the other. However, the experimental observation of some methylcyclopentane byproduct in some systems seems to suggest the formation of at least some alkyl hydride intermediates (Scheme 6). A computational study on the formation of methylcyclopentane confirms that this route is a side reaction and that direct H-transfer is the main reaction path.43 Following the confirmation of a metallacyclic mechanism for the selective trimerization, a more general mechanism for oligo- and polymerization has been suggested as an extension, as indicated in Scheme 5. While there is good evidence that the reductive elimination from the metallacyclopentane is significantly slower than for larger cycles, there should be no significant discrimination between the larger cycles. Thus, in cases where insertion of ethylene into the larger metallacycles becomes competitive or even faster than elimination, oligomerization or even polymerization should be observed. This was confirmed by the C2H4/C2D4 test for catalysts producing oligomers beyond the trimer. The same test has so far not been reported for the heterogeneous Phillips catalyst itself, although a similar test using deuterated a-olefins as chain length limiting reagent seem to support a non-metallacyclic mechanism for at least some active sites of the Phillips catalyst.44 On the other hand, a good match of the end group distribution in the Phillips polyethylene and those in C10 co-trimer fraction (1-hexene þ 2 ethylene) of a trimerization catalyst suggests that metallacycles are involved at least in the end group formation of the Phillips catalyst, as well.45 Thus, the final verdict on the role of metallacycle in the Phillips catalyst is still unresolved. Nearly 10 years ago, it was surprisingly discovered that a small modification to the ‘PNP catalyst’ 2 shown in Figure 2 (no MeO groups) leads to a highly selective catalyst for tetramerization of ethylene to 1-octene. Systematic variations on the ligands showed that the C6/C8 selectivity depends on both steric effects and donor ability, with less bulk and less donation favoring tetramerization. Similarly, the weaker the coordinating anion is, the more tetramer is formed. So far, these observations could not be satisfactorily rationalized by computational studies, as differences between chromacycloheptanes and chromacyclononanes are small. An alternative explanation for the tetramerization selectivity has been provided by Rosenthal, as shown in Scheme 7.46 The dinuclear route would not require metallacycles beyond the metallacyclopentane and would readily explain the lack of oligomers higher than octene. Addition and insertion of

CrLn H

CrLn - CrLn H

Oligo- and Polymerization of Olefins

ethylene units resulting in similar (
C2H4–)n bridges between two chromium surface sites in the Phillips catalyst has been proposed as another possible initiation pathway without the need for existing metal–alkyl bonds. Detailed kinetic analyses on this systems found unusual broken order n for the dependence on ethylene pressure ([ethylene]n). Several studies found orders n ¼ 1.6–1.7 – but different dependences for tri- (n ¼ 1.3) and tetramerization (n ¼ 2.05). Again, this might suggest a fundamentally different mechanism for tri- and tetramerization. The much slower trimerization of a-olefins was found to be first order in monomer, suggesting that the insertion of the third olefin is the ratelimiting step in this system.38 Similarly, first-order dependence on ethylene was found for selective trimerization using titanium catalysts. Recent results on pyridine chromium complexes bearing two NHC carbene donor arms show that a-olefins are predominantly dimerized via a metallacyclic mechanism. Thus, metallacycles can also support dimerization.47

6.06.2.1.4 Chain termination steps 6.06.2.1.4.1 b-H elimination Metal–alkyl complexes with an accessible vacant coordination site can undergo b-H elimination (Scheme 1) to give olefin hydride complexes. The olefin is usually susceptible to associative ligand exchange, with monomer olefin leading to polymerization chain termination. The transition state of b-H elimination can be probed by the kinetics of the reverse insertion of olefins into metal hydrides, for example, for metallocene dihydrides.48 6.06.2.1.4.2 b-H transfer to new monomer Metal–alkyl olefin complexes are the either short-lived (early transition metals) intermediates or long-lived resting states of polymerization catalysts prior to olefin insertion. However, this complex can also undergo b-H transfer from the alkyl to the coordinated olefin (Scheme 1) to give another alkyl olefin complex with the long polymer chain in the olefin. Associative ligand exchange with monomer olefin also leads to polymerization chain termination, as in the case of b-hydrogen elimination. This is apparently the dominant chain termination mechanism under the usual experimental conditions for both R⬘ N Cl S Cr S R Cl R Cl

R N N R Cr Cl Cl Cl

Ar N P Ar P Ar Cr O Cl Cl Cl

R N

3

2

1

LnCr

CrLn

Ln Cr

early49 and late transition metals,33 as found by DFT calculations. 6.06.2.1.4.3 Reductive b-H þ alkyl elimination A special variation of the b-hydrogen elimination/transfer has been observed for dialkyl complexes (especially metallacycles). Oligomer or polymer growth in dialkyl complexes can be terminated by transfer of a b-hydrogen from one alkyl to the terminal carbon atom of the other, resulting in an olefin (complex) and alkane under reduction of the metal by two (formal) oxidation states. This is the main chain transfer process in the metallacyclic mechanisms shown in Scheme 5 (see Section 6.06.2.1.3.4). 6.06.2.1.4.4 Chain transfer to other reagents50 An alternative mechanism for chain termination is chain transfer to other reagents, often to other metals in the solution. The most frequently used additional metal used in polymerization is aluminum, found in the co-catalyst. Aluminum alkyls (often AlMe3 present in MAO – see Section 6.06.5.1) can act as a receiving reagent for the polymer chain, as shown in Scheme 8.51,52 The majority of examples for chain transfer to aluminum are reported in zirconocene-mediated propylene polymerization. The polymeryl-aluminum compound remains in solution until acidic work-up produces saturated chain ends. The lack of unsaturated end groups is often a good indication that chain transfer to aluminum has occurred and is best detected by 13C NMR spectroscopy of the isolated polymer. Another indication for transfer to aluminum is a decrease of polymer molecular weight, with increasing aluminum co-catalyst concentration. Numerous examples for chain transfer to aluminum have also been found for non-metallocene polymerization catalysts of both early and late transition metals. Similar chain transfer to other metals or reagents can be achieved by the addition of appropriate reagents. In these cases, the reagents have been added on purpose to obtain the corresponding reagent-terminated polymers. These reagents include silanes RSiH3, boranes R2BH, ZnEt2, or MgR253 and even phosphanes R2PH, to yield polymers terminated with SiRH2, R2B, ZnR, MgR, or P(¼O)R2, respectively. The remaining Si–H groups in silane terminated polymers allow further cross-linking to other polymer chains by the same process and can significantly affect the molecular weight and the properties of the polymer. Oxidative work-up of the borane terminated polymers leads to terminal alcohol functionalization. 6.06.2.1.4.5 Living polymerization A growing number of polymerization catalysts show no chain termination reaction, and the growing polymer chain stays

Figure 2 Selection of pre-catalysts active in selective ethylene trimerization.

4

133

Ln Cr

Scheme 7 Proposed bimetallic mechanism for selective tetramerization.46

Ln Cr

Ln Cr

1-Octene

134

Oligo- and Polymerization of Olefins

P

P LnM Me AlMe2

LnM

Me

P +

AlMe2

+ AlMe3 P

Me

P

Scheme 8 Chain transfer to AlMe3.

attached to the polymerization catalyst until the polymerization is typically quenched by the addition of aqueous or alcoholic acid. Living polymerization offers a number of advantages. The chain length or molecular weight can be controlled by the ratio of monomers to polymerization active catalysts, which may be different from the amount of catalyst added if activation is slow or incomplete. As the catalyst remains active after consumption of all monomer, different monomers can be added in sequence, leading to block copolymerization (see Section 6.06.3.3).54

6.06.2.2

P +

+ H+ +n

LnM

LnM

L nM

Olefin Metathesis (Carbene) Mechanisms

Polymerization via olefin metathesis was discovered only a few years after ZN coordination polymerization, when the catalytic behavior of several metals was explored with respect to cyclic olefins, although the mechanistic understanding of this fundamentally different kind of catalysis only emerged in the 1970s. Olefin metathesis via organometallic carbene complexes has become an important toolbox in organic synthesis, especially with the introduction of highly active, yet surprisingly stable ruthenium-based catalysts by Grubbs and co-workers nearly 20 years ago.55 The performance has been gradually optimized over the years and the introduction of heterocyclic carbene ligands has, in particular, led to a versatile toolbox of metathesis catalysts.56 The catalysts are tolerant to a variety of functional groups and can be applied to catalyze the polymerization of olefins via a mechanism fundamentally different to standard coordination polymerization, giving access to completely new polymer architectures. Olefin metathesis is covered in Chapter 6.05; as such, only its application to olefin polymerization will be covered here. As a consequence of the metathesis mechanism, all polymers produced contain at least one C–C double bond per monomer, allowing further postpolymerization functionalization. The metal–carbene bond in metathesis catalysts is often very stable, with little or no competing chain transfer reactions. Thus, metathesis polymerization is usually living. Hydrogenation of the polymer may lead to the same saturated polymers, as with other polymerization catalysts. The enormous general impact of metathesis on chemistry has been signaled by the Nobel Prize awarded in 2005 to its key researchers: Yves Chauvin, Richard Schrock and Robert Grubbs.

6.06.2.2.1 Ring Ring-opening metathesis polymerization The main application of olefin metathesis utilizes the opening of cyclic olefins, as shown in Scheme 9.

Extended P LnM

P

P

+n

LnM

n+1 Scheme 9 ROMP mechanism.

The driving force for the polymerization is typically the release of some ring strain in the cyclic monomer, leading to an enthalpy driven polymerization. One of the earliest commercial applications of ROMP was the polymerization of dicyclopentadiene, as shown in Scheme 10. Other monomers like cyclopentene or cyclooctenes are typical examples of monomers with ring strain due to smaller or larger than optimal ring size. Consequently, cyclohexene with minimal ring strain is usually unaffected by ROMP catalysts. The increasing tolerance of new ROMP catalysts has opened the way for ever more complex functionalized polymers, and the living nature of ROMP gives easy access to block copolymerization utilizing different functionalization from different monomers. The tolerance to aqueous solvents has allowed polymerization in biphasic systems, with additional influence on the polymer structure on the phase boundary, for example, by controlling polymer size by droplet size. This has led to an enormous variety of new polymers and new applications, and it has been reviewed extensively.57,58 A subset of ROMP is entropy-driven ROMP (ED-ROMP).59 While regular ROMP is enthalpy-driven by some ring strain in the monomer, virtually strain-free macrocycles (above about 14 ring atoms) can also be polymerized due an increase in entropy upon ring opening. As the entropy gain on ring opening competes with the concentration-dependent loss of translational entropy, ideal ED-ROMP reactions are carried out under high monomer concentration, ideally solvent-free. Due to the large ring size, most functional groups imaginable can be incorporated into the monomer macrocycle and, subsequently, into the polymer main-chain, in addition to the variety of functionalized polymers already achieved by regular ROMP.

6.06.2.2.2 Acyclic diene metahesis polymerization60 Olefin polymerization based on metathesis does not require cyclic monomers. One alternative mechanism is the step wise cross-metathesis of acyclic a,o-dienes, with ethylene release driving the reaction, as shown in Scheme 11. While the concept of acyclic diene metahesis polymerization (ADMET) has been pursued since the 1970s, the experimental discovery of the reaction by Wagener and co-workers did not occur until 1987.61 The main problem was finding a catalyst that would only give metathesis and no other catalytic process

Oligo- and Polymerization of Olefins

ROMP

135

n

n Cross-linking

Scheme 10 Cross-linkable ROMP polymer from dicyclopentadiene.

n times Spacer

+ LnM

Non productive

+

n

Spacer

n

Spacer

Spacer

LnM Spacer

Spacer

+

or

MLn

Spacer

+ MLn

Spacer Spacer

Spacer

Scheme 11 ADMET mechanism.

Bu n

ADMET n

−

Bu n

H2 n

m

Bu n

n

m

Scheme 12 Polyethylene with well-defined branching via ADMET.

that would convert the terminal olefins, and many active metathesis catalysts are similar to active insertion polymerization catalysts. One useful test for a suitable ADMET catalyst is reaction with styrene, giving stilbene and no polystyrene. There is a regiochemical requirement for productive metathesis. The sterically favorable a,a0 -metallacyclobutane intermediate A leads only to non productive metathesis and only the less favorable a,b-metallacyclobutane intermediate B can lead to coupling between two monomers. Thus, monomers with little steric demand around the olefins are used to facilitate intermediate B and to achieve faster polymerization. A second regiochemical outcome of the metathesis regards the relative orientation of the metallacyclobutane substituents. Usually a trans configuration is favored leading to trans double bonds. ADMET is a step-growth, condensation polymerization, fundamentally different from either ROMP or coordination polymerization in that the growing polymer does not stay attached to the metal. High conversion is achieved by removal of the small metathesis by product, usually ethylene. As for all condensation polymerizations, high conversion is necessary to reach high-molecular-weight polymers. The negative effect of cyclization reactions ia also typical for condensation polymerization. This ‘backbiting’ cyclization is favored under high dilution. Thus, polymerization in bulk or at high concentrations is typical for ADMET, along with the use of monomers that do not form favorable ring sizes.

Any unsaturation in polymers produced by ADMET can be hydrogenated to yield polyethylenes with precise linearity and positioning of ‘functional’ groups, as shown in the example in Scheme 12, with a polyethylene that contains butyl branches precisely on every seventy-fifth carbon atom of the PE chain.62 Such precise polymers have been invaluable for the NMR characterization of polymer branches and the study of branching’s influence on polymer properties. Metathesis of the vinyl end groups with functionalized olefins can lead to polymers with precise end-functionalization. A subset of ADMET is ring expansion metathesis polymerization (REMP), where the catalyst is a metallacyclic carbene leading to cyclic polymers, as outlined in Section 6.06.3.4.

6.06.2.2.3 Green–Rooney mechanism An alternative mechanism, shown in Scheme 13, combining the olefin metathesis with the coordination polymerization, has been proposed by Green and Rooney.63 The mechanism requires an increase of the formal oxidation state by two and can only operate for metal complexes with two or more d-electrons. Convincing evidence against this mechanism operating in metallocene catalysts has been reported by Grubbs.64 However, the proposal led to the modified form by Brookhart and Green involving a-agostic hydrogen rather than complete a-hydride migration.

136

Oligo- and Polymerization of Olefins

6.06.2.3

Cationic Polymerization65

Olefins capable of stabilizing carbocations after reaction with acids can be polymerized by repeated addition of further olefin monomers to this carbocation. Thus, this cationic polymerization does not require any metal complexes, and a number of Brønsted or Lewis acids have been used for initiation. However, many of the highly Lewis acidic metal centers generated in the activation of coordination polymerization are also excellent initiators for cationic polymerization, and several metallocene catalysts have also been used for cationic polymerization, for example, zirconocene hydrides with trityl salts of weakly coordinating anions.66 The typical requirement for weakly coordinating anions in coordination polymerization also enhances cationic polymerization. Thus, in cases of the polymerization of olefins susceptible to cationic polymerization a detailed mechanistic investigation may be required to decide whether cationic or coordination polymerization is dominant. Only recently has the structure of the first disubstituted olefin coordination to [Cp2ZrMeþ] (complex 4 in Scheme 14) been obtained, and it shows a very unsymmetrical ‘olefin coordination’ with a short (2.61 A˚) Zr–C bond and much longer (3.36 A˚) distance to the carbocationic second carbon atom.14 As shown in Scheme 14, there are important mechanistic differences to coordination polymerization. The active cationic center moves away from the metal center after the initiation step. Thus, the ligands cannot assert any stereo selectivity beyond the first few insertion steps. On the other hand, the anion stays close to the active cationic center and the activity depends on the coordinating ability of the anion similar to coordination polymerization catalysts. One of the ligands at the metal can be an alkyl, but no metal–alkyls are required for cationic polymerization. Indeed, ‘over-activation’ of metal dialkyl complexes may lead to dications with even higher activity for initiating cationic polymerization. Even completely saturated simple [M(MeCN)6]2þ

H H

H

H

P M

H

H

P

P

M

M α-H shift

(Extended P)

H

H

H

H

P

P M

M

Scheme 13 Green–Rooney mechanism.

complexes have been used as initiators. In some cases, small amounts of water were found to have a beneficial effect, possiblly as a source of a Brønsted acid, though the water’s exact role remains unclear. As the role of the metal center in cationic polymerization is to provide Lewis acidity, any Lewis acidic co-catalysts added can be the source of the initiation as well. Indeed, some chelating diboranes or MAO used as co-catalysts for coordination polymerization have been reported to initiate cationic polymerization of isobutene without any other added metal complex. Another difference between coordination and cationic polymerization is the typical monomer substrate. Cations generated from ethylene or a-olefins are usually not stable enough to sustain cationic polymerization and additional stabilization of the carbocation is required. On the other hand, tertiary, allylic or benzylic carbocations lead to effective polymerization and typical substrates for cationic polymerization are isobutene, isoprene or styrene. Styrene is a typical borderline monomer that could be polymerized by either cationic or coordination polymerization.

6.06.2.4

Anionic Polymerization67

Olefins capable of stabilizing an anionic charge upon addition of an anion can be polymerized by an anionic pathway. Such olefins are styrenes, butadienes or functional olefins like methyl methacrylate. Initiators are carbanions like nBuLi, t BuLi, or (still better) sBuLi, or they are generated from alkali metals, often via reaction with naphthalene and subsequent radical-anion dimerization, as shown in Scheme 15. The role of the metal is mainly to provide a charge balance without direct involvement in the polymerization steps. However, the activity depends strongly on aggregation, which is influenced by the metal used as well as the ligand donors added. The anionic polymerization can be retarded in order to gain more control by the addition of magnesium or aluminium compounds.68 The addition of organometallic Lewis acids (R2Zn, R3B, or R3Al) can have beneficial effects on the stereo control of the anionic polymerization of polar olefins. These main group alkyl compounds form stable ‘ate’ complexes, with the initiating alkyl lithium reagent leading to slower reactions, and allow anionic polymerization in bulk styrene, for example, without overheating the reactor. Sodium hydride coupled with AlR3 has become a low-cost alternative to alkyl lithium, with greater control over the styrene polymerization. Anionic polymerization of (meth)acrylates can also be initiated by methyl porphyrinatoaluminum under irradiation with visible light or even by some metallocenes of group 3 and 4. Anionic polymerization can be living in the absence of any proton sources. Thus block-copolymers can be produced

n [LnM]+ A- +

LnM

A-

LnM

LnM = Cp2Zr: 4 Ph Other monomers: Scheme 14 Cationic olefin polymerization.

Ph

Ph

Ph

n

A-

Oligo- and Polymerization of Olefins

including interesting architectures like star-block copolymers when central units with several initiation sites are used.

6.06.2.5

Radical Polymerization69

Radical polymerization of ethylene initiated by oxygen at high pressure and temperature was first ethylene polymerization process introduced in the 1930s (ICI). The process allows little control over the propagation, and highly branched low-density polyethylene with broad molecular weight distribution is produced. The reason for the lack of control is the slow generation of primary radicals (about 10
5 s
1) relative to fast propagation (about 103 M
1 s
1). After a few thousand addition steps, the chain is terminated within about a second, allowing little time to manipulate the growing chain. Therefore the much better control in coordination polymerization dominated olefin polymerization up to the 1990s. However, radical polymerization can be turned into a controlled process if the rate of initiation is faster than propagation and the concentration of active radicals is kept low to prevent termination by radical combination while the concentration of growing chains is still high. These conditions can be met by the introduction of dormant chains in a dynamic and fast equilibrium with an active radical species. The stability of many transition metal radicals makes suitable transition metal complexes the ideal catalysts for radical polymerization, by providing stable dormant radical species. This way, the polymer grows only for a small fraction of a second each time when active, while the growing chain remains alive for hours in the dormant state, repeatedly returning briefly to active form every minute or so. Thus, the growing chain is active long enough to change the monomer for block-copolymerization. There are two main ways of using metal complexes to stabilize dormant species, as shown in Scheme 16. The first uses the susceptibility of some organometallic M-alkyl bonds to reversible homolytic cleavage in order to generate the active radical chain (organometallic mediated radical polymerization, OMRP). Alternatively, the polymer chain can be terminated by an active group X that can be

reversibly abstracted as a radical by a suitable metal complex in order to generate the growing polymer radical. The group (or atom) X is transferred during the activation process; hence, it is called atom transfer radical polymerization (ATRP). In both cases, the metal undergoes a reversible one-electron process. Thus, catalysts must have suitable redox potential, as well as fast and reversible redox chemistry. These requirements are similar to those found in biological electron transfer enzymes (see Volume 3). Thus typical catalysts contain porphyrin complexes of Fe or Co or complexes of copper. Indeed, some enzymes have been used to catalyze radical polymerization.70 Olefin monomers of radical polymerization are typically those with radical stabilizing groups, for example, methacrylates or vinyl acetates but also styrene.

6.06.2.5.1 Organometallic radical polymerization71 Typical complexes used for OMRP are shown in Scheme 17. In addition to radical combinations, another chain termination step is b-hydrogen elimination, similar to coordination polymerization. Thus, a polymer with an unsaturated end group is formed. The metal hydride will insert one monomer unit to generate an organometallic complex, starting a new polymer chain by radical polymerization. The b-hydrogen transfer can be inhibited by introducing more steric hindrance around the metal. Titanocenes and monocyclopentadienyl titanium complexes are good OMRP catalysts via the Ti(III/IV) cycle. Thus, the same catalysts used for coordination polymerization may catalyze radical polymerization after activation with a reducing agent (e.g., Zn) and a radical generating reagent, for example, an epoxide, as shown in Scheme 17. If the metal has additional coordination sites available, an exchange transfer of the dormant polymer chain with an active radical chain is possible (degenerative transfer), for example, Co(acac)2 polymerizes vinyl acetate via degenerative transfer only in the absence of Lewis bases. Addition of a single

LnM + P

LnM – P

OMRP:

Dormant state ATRP: 2

2M+2

137

+ Monomers Propagation + Monomers

LnM-X + P

LnM + X – P

Scheme 16 OMRP and ATRP mechanisms.

M

M = alkali metal +2

Ph

Ph Ph Ph

Ph

2

Ph + (m+n)

Cl Ph

Ph

M

Ti Cl

Ar

N

Cl

Cr

Co

N Ar

N

R Cp2TiIIICl + O

Ph

n Ph Ph Ph Ph Ph

m Ph Ph

Scheme 15 Anionic olefin polymerization.

Cp2TiIIICl + P

O N

2 Cp2TiIIICl + ZnCl2

2 Cp2TiIVCl2 + Zn

Ph

Ph O

Ph

Cp2TiIV O

Monomers P R

Cp2TiIV

Scheme 17 Typical OMRP catalysts.

Cl P

138

Oligo- and Polymerization of Olefins

pyridine per cobalt blocks one coordination site and, subsequently, the degenerative transfer.

(activator regenerated by electron transfer, ARGET) solved these problems, and controlled ATRP polymerization is now possible at very low metal concentrations.

6.06.2.5.2 Atom transfer radical polymerization72 ATRP, first reported in 1995 by Sawamoto (Ru)73 and coworkers and by Wang and Matyjaszewski (Cu),74 often results in living polymerization, allowing block-copolymer formation and functionalization at the living polymer end.75 As shown in Scheme 16, ATRP requires an atom X to be transferred between the growing polymer chain and the metal. This transfer occurs via a reversible inner-sphere electrontransfer process. The transfer atoms X are typically halogens or pseudo-halogens. As the organometallic radical is much more stable than the free radical, the equilibrium is greatly shifted to the dormant metal polymer complex and the concentration of free radical species is kept very low. Thus, the rate constants for activation and deactivation and their ratio determine the overall rate of polymerization as well as the polydispersity. Both rates should be high, and deactivation should be much faster than activation. The KATPR (¼kact/ kdeact) equilibrium can be broken down into a product of Polymer–X bond and M–X bond homolysis equilibrium constants, thus a metal- and an X-dependent term. The dependence of polymerization activity on the ‘halogenophilicity’ of the metal has been confirmed experimentally in several cases. The catalyst must not only have the right redox potential between two oxidation states separated by one electron but also have a reasonable affinity to the exchanging group X. The reduced form of the complex must have an available coordination site for X. One typical metal for ATRP is copper, but many other metals have been employed as detailed in the metal specific Section 6.06.4. ATRP requires an initiating reagent RX. If initiation is fast relative to transfer and termination, the concentration of growing chains will be equal to the RX concentration and polymerization is first-order in RX. Molecular weight control depends on fast transfer of X. Best results are usually obtained with bromides or chlorides. Iodides have also been used, but fluorides have C–F bonds too strong to allow ATRP. Other halogen radical generating initiators such as CHCl3 or N–X, S–X, or O–X compounds have been used. Solvent effects are relatively minor for the Cu ATRP systems studied in detail and range over less than two orders of magnitude. One exception seems to be water, where ATRP rates have been found to be three orders of magnitude higher than in organic solvents, with a much higher radical concentration in water. Thus, ATRP is very tolerant of most commonly used solvents. Olefin monomers require some radical stabilizing groups; as such, (meth)acrylates, (meth)aryl amides, and acrylonitriles have been used, as well as styrene. Thus, styrene is a substrate that can be polymerized by all mechanisms presented above. In ATRP, the catalyst is usually introduced in its reduced (and air/moisture sensitive) form. In reverse ATRP, the catalyst is introduced in its oxidized form and reduced by another radical initiator. However, this way, the atom X is introduced with the catalyst and the M:X ratio cannot be independently reduced to allow block copolymerization. The development of suitable reducing and activator regenerating reagents

6.06.3

Control Over Polymer Structure

The properties of polymers strongly depend on the structure. Thus, control over the polymer structure is important to tailor the product for the properties required.76 The first ethylene polymerization by the free-radical, high-pressure ICI process in the 1930s produced a highly and randomly branched polymer with poor polymer properties: low-density polyethylene (LDPE). ZN and Phillips catalysts developed in the 1950s led to highly linear polymers (high-density polyethylene, or HDPE) and some stereo-control for polypropylene. In subsequent decades, increasing control over copolymerization between ethylene and a-olefins allowed the development of linear polymers with well-defined side chains, linear lowdensity polyethylene (LLDPE). The ‘metallocene revolution’ in the two decades following Kaminski and co-worker’s discovery in 197577 led to an unprecedented control over the polymer structure, including stereo control of poly(a-olefins) by tuning of the cyclopentadienyl substituents and co-catalysts. The last two decades saw the development of post-metallocene technology, which expanded polymerization control using easily variable non-cyclopentadienyl ligands combined with high-throughput technologies for the catalyst discovery. Living polymerization catalysts have been developed that allow access to block-copolymerization with fascinating new properties. Expansion to mid- and late-transition metal catalysts opened the way to new polymer structures and the inclusion of functional monomers. In addition to control over the molecular weight, catalysts have been developed for selective oligomerization, that is, selective tri- and tetramerization.

6.06.3.1

Linear Oligo- and Polymers

6.06.3.1.1 High-density polyethylene Linear HDPE was first prepared by Alex Zletz of Standard Oil in 1950 using reduced molybdenum on alumina. However, the value of the discovery was not realized until much later, and HDPE technology was developed and shaped based on the Phillips and Ziegler catalysts (discovered in 1951 and 1953, respectively).78 In theory, HDPE is an infinite chain of methylene groups (CH2)n; in practice, many variations exist, differing by chain length and molecular weight distribution, end groups and ‘errors’ that result in some branching. Thus, the different HDPE catalysts produce polymers with different properties.

6.06.3.1.2 Stereo and tacticity control of a-olefin polymers79 Polymerization by regular (1,2-) insertion of a-olefins can lead to three principal forms of polymer, as shown in Scheme 18. The orientation of an alkyl branch relative to the next branch is either meso (m) or racemic (r). If all branches have the same orientation (all m), the structure is isotactic. If the orientation is alternating (all r), the structure is syndiotactic and a random orientation leads to atactic polymer. In addition, there are intermediate forms, for example, hemi-isotactic, with

Oligo- and Polymerization of Olefins

C2 symmetric

Cs symmetric

C1 symmetric

P

P

No control over polymer orientation

Me P

P

M

M

P

P

Zr +

Insertion

M

+

Site epimerization

+

+

Me Isospecific

Zr+ M

Me P M

M

P M

P

n Isotactic

P

Me Unspecific

Zr +

M

P

139

P

Zr+

Insertion (atactic)

Scheme 19 Role of site-epimerization in stereo-control.

n Syndiotactic

n Hemi-isotactic

n Atactic

R1 N M O

R2

Scheme 18 Stereo-control of propylene polymerization with metallocenes.

R3 Figure 3 Phenoxy-imine ligand used in FI catalysts.

alternating insertions under iso-control and no control. These different forms of the polymer (especially polypropylene) have very different properties. The original work by Natta and coworkers on the selective production of each of these forms by catalyst modifications has given preparative access to these forms. However, the well-defined homogeneous single-site metallocene catalysts opened the way for rational catalyst design 30 years ago, and the general dependence of polymer structure on the symmetry of the metallocene, shown in Scheme 18, has become chemistry textbook material. The structure of polypropylene can be studied in particular detail by 13C NMR spectroscopy. Stereo errors relative to two (pentades) or more monomer units to either side can be resolved and used as a ‘record’ for the mechanism of the stereo regulation. An example is shown in Scheme 19.80 This example shows how the isotacticity increases in a C1 symmetric complex when the rate of insertion is slowed down by lower propylene concentration. This observation can be explained by relatively faster site epimerization back to the less hindered and more stereo selective site for the growing polymer chain after each insertion. If site epimerization is negligible, propylene is inserted with alternating iso- and unspecific orientation, resulting in hemi-isotactic polypropylene. The variation of the steric and electronic environment of the metal by substituents on the metallocene is limited and often requires difficult multi-step organic synthesis. The discovery of highly active non- or post-metallocene catalysts two decades ago allowed further catalyst development beyond these limitations, often with much simpler ligand synthesis. Particularly successful post-metallocene ligands are phenoxyimines (FI catalysts), shown in Figure 3. The ligand has been particularly successful in group 4 catalysts, though it has been successful with many other transition metals, as well. The phenoxy group provides strong

bonding to the metal, and the more flexible imine allows a tuning of the electronic properties. Substituents on both the phenoxy arene and the imine allow a precise tuning of the steric environment of the metal, including chirality for two of these ligands. Linking the two ligands via the N atoms leads to a more rigid class of complexes containing [ONNO] ligands. The FI type catalysts have been reviewed recently.81 Group 4 complexes achieve ultrahigh activity for ethylene polymerization and give living syndiotactic propylene polymerization. They also show high activity for higher a-olefins with often limited stereo- and regio-selectivity. Bridged [ONNO] type ligands give higher selectivity, for example, highly isotactic poly(1-hexene) has been reported, though activity is not very high due to the strong N-donor atoms. More weakly coordinating S- instead of N-donors lead to increased activity and [OSSO] type ligands have been reviewed.82 The introduction of non-metallocene catalysts has opened the way to explore different symmetry forms. As with C2symmetric metallocenes, threefold ligand symmetry (C3) can lead to isotactic poly-a-olefins.83 An example is the highly active scandium catalyst, shown in eqn [1], which gives highly isotactic poly(1-hexene) (90% mmmm).

O

N

N Pri Me3Si

Sc

N

O O

+ [Ph3C+] [B(C6F5)4-]

Pri Pri - Ph CCH SiMe 3 2 3

SiMe SiMe3 3

O

N

N Pri

Sc

N

O O Pri Pri

SiMe SiMe3 3

[B(C6F5)4-]

½1 The complex interplay of enantiomorphic site versus chain end control (including the effect of chiral monomers) has been studied in detail by Bercaw and co-workers.84

140

Oligo- and Polymerization of Olefins

Stereo control in non-metallocene catalysts is much more difficult to predict. While 1,2-insertion of propylene is the preferential mode of insertion for metallocenes, a large variety of possible insertion modes and stereo chemistries have been observed for non-metallocenes, resulting in complex polymer structures. Results on the stereo selectivity of non-metallocene catalysts using isotope labeled 13C NMR studies have recently been reviewed.85 The stereo-selectivity of the olefin polymerization can be used to resolve chiral olefins by preferentially polymerizing one enantiomer.86

6.06.3.2

Chain walking leads to catalyst deactivation by b-X elimination when olefins contain some functionality X (halide or OR). On the other hand, chain walking cannot proceed past quaternary carbon atoms in olefins.

6.06.3.2.2 Branched polymers from copolymerization A controlled way of introducing branches of a specific length is the copolymerization of ethylene with a-olefins. The branches break up the crystallinity of linear polyethylene. Therefore, control of the branching allows the tuning the thermal polymer properties. This linear low-density polyethylene (LLDPE) has become one of the three important industrial forms of polyethylene, with a market share of around 30%.78 Many transition metal catalysts have a much higher polymerization rate for ethylene than for higher a-olefins, making it difficult to incorporate the latter. Complexes of less steric hindrance to olefin coordination tend to have more similar rates and become suitable to copolymerization. An important class of catalysts for copolymerization are the so-called ‘constraint geometry catalysts’ (CGC), shown in Figure 5. The major co-monomers are 1-butene, 1-hexene and 1-octene. The high demand for 1-hexene for copolymerization has led to the development of selective trimerization of ethylene into an industrial process, and a selective tetramerization process may soon follow.

Branched Polymers and Copolymers

One important parameter for polymers is the branching. Control over the architecture of branching allows extensive control over the polymer properties. This can be achieved by either building branches during the polymerization or introduction of branches with branch containing monomers in copolymerization. Control over branching in sections of the polymer leads to block-copolymers.

6.06.3.2.1 Branched polymers from homo-polymerization An important mechanism for the introduction of branching is the chain walking polymerization (CWP). If b-hydride elimination and re-insertion is fast relative to chain transfer or monomer insertion, the metal center can ‘walk’ down the polymer chain, introducing a branch at the site of the next monomer insertion. Early transition metals tend to have fast monomer insertion and allow little, if any, chain walking. Late transition metals tend to have fast b-hydride elimination, usually coupled with fast chain transfer. In 1995, Brookhart and co-workers discovered that steric blocking of the axial sites in planar Ni(II) and Pd(II) a-diimine complexes can prevent chain transfer. However, extensive chain walking can sometimes still occur, leading to branches during the polymerization (Scheme 20).4 The degree of branching can be controlled by the rate of insertion relative to chain walking. According to kinetic studies of the catalysts, this ratio is proportional to the ethylene concentration (pressure). Electron withdrawing ligand substituents can also increase the branching.87

6.06.3.3

Block-copolymerization

Block copolymers are polymers containing two or more homopolymer blocks, leading to specific polymer properties, mainly due to micro-phase separation of the blocks, which results in periodic nanostructures within the polymer. A type of living polymerization catalyst will be able to produce block copolymers by the sequential addition of different monomers. However, this methodology results in only one polymer chain per catalyst molecule, and only few commercial processes (e.g., SBS elastomers, Kraton) use this inefficient method. One solution to this problem are catalysts with two different catalytic centers, resulting in blocks of two types of polymer structure of the same monomer (for example, hard blocks of

P

+ (longer P) – propagation

R N + M N R

P Insertion

R N + M N R

Steric bulk (R) in axial positions (resting state)

H ‡

H transfer

P R N + M N R

R N + M N H R

P

H P

Chain transfer Scheme 20 Branching through chain walking.

P H

+

+ (Longer P) R N + M N R

R N + M N R

R N + M N R Chain walking Branched polymer

P

Oligo- and Polymerization of Olefins

regular and soft blocks of irregular polypropylene) or blocks with two different co-monomer ratios (for example, hard blocks of linear polyethylene and soft blocks with high aolefin co-monomer).88 This can be achieved by simply combining two different catalysts89 or by using a single catalyst that switches between two conformers with different catalytic performance (Scheme 21).90 In the case of two different catalysts, a chain shuttling agent (CSA, e.g., ZnEt2) can be used to combine the sections produced at the two catalyst sites into a single polymer chain (Scheme 22).

Cyclic Polymers91

6.06.3.4

The different topology of cyclic polymers leads to fundamentally different polymer properties, as they lack any chain end. Synthetic methods for ring closure after polymerization are

limited by low efficiency and small accessible ring sizes, and they cannot exclude entanglement.92 These problems have recently been solved for polyolefins by ring-expansion metathesis polymerization (REMP) followed by hydrogenation of the remaining unsaturation.93 The important feature of the mechanism, illustrated in Scheme 23, is that the polymer chain end remains attached to the ruthenium center as a carbene complex. Cyclic olefin monomers keep inserting into this carbene until intra-molecular metathesis with a double bond within the metallacycle leads to the formation of a cyclic polymer containing multiple double bonds. These can either reinsert in competition with the cyclic olefin monomer or remain until all monomer is consumed. The ratio of these two alternatives can be influenced by the ring size of the initial catalyst. Catalysts with six carbon tethers tend to have little reinsertion, leading to a molecular weight evolution profile resembling a chain growth mechanism, while five carbon tethers lead to the ready release of five carbon tethered complexes with multiple reinsertions, resulting in polymerization resembling a step-growth mechanism.

P Zr

P Zr

6.06.3.5

Slow relative to insertion

Many

Control of Molecular Weight

Generally, the average molecular weight of a polymer is the result of the relative rate of chain propagation to chain termination. When propagation is much faster than termination, high-molecular-weight polymers are obtained. In the extreme case of negligible chain termination (living polymerization), the molecular weight will be limited by the amount of polymer available per catalyst. In the other extreme of fast chain

Many

Isotactic blocks

Atactic blocks

Scheme 21 Isotactic–atactic block polymers via slow reversible isomerization.

Hard HDPE Cat1 +

ZnEt2

Cat1 + Cat2 R

Scheme 22

141

Zn

Cat2 Soft copolymer

Hard/soft multiblock copolymer

Hard/soft multi-block polymers via chain shuttling.

N N

N Cl

N Ru

Ru Cl PCy3

N R

N Cl

Ru Cl PCy3

R

R

(n−1)

N

N Ru

R

n−1 H2 Cyclo-(CH2)8n

n

N

N Cl

Ru Cl PCy3

R

n Scheme 23 Mechanism of REMP, Cy ¼ cyclohexyl.

142

Oligo- and Polymerization of Olefins

O

Ph2 P Ni O

-COD O

Ph2 P Ni H O

O

H

Ph2 P Ni O n

O

n

Ph2 P Ni O

n

Scheme 24 SHOP mechanism, COD ¼ 1,5-cyclooctadiene.

termination, low-molecular-weight dimers or oligomers are produced. The distribution of oligomers follows a regular Flory– Schulz distribution as long as the propagation and termination rate constants are similar for all metal oligomer complexes. If the latter is not case, selective oligomerization to a specific chain length may be achieved (see Section 6.06.3.5.3). In the case of coordination polymerization, chain termination is dominated by b-H elimination leading to olefin complexes. As olefin complexes are more stable for late transition metals, chain termination becomes faster for late transition metals, leading to a trend for high-molecular-weight polymers for early transition metal complexes and oligomerization for late transition metal complexes.

6.06.3.5.1 Oligomerization One typical example of the use of late transition metals in ethylene oligomerization is the catalyst for the Shell Higher Olefin Process (SHOP) (Scheme 24). The SHOP process was commercialized in 1977 and annual global production capacity reached 106 tons, producing the commercially valuable a-olefins from C6–C18 and subsequent conversion to alcohols. The simple concept for understanding the polymerization differences between early and late transition metals based on the stability of olefin complexes overlooked the fact that an associative olefin exchange with monomer is required as a last step for chain termination. This step is fast for complexes like the one used in SHOP, but Brookhart94 and co-workers discovered 20 years ago that steric blocking of the axial positions in square planar a-diimine nickel and palladium complexes slows down this olefin exchange to such an extent that highmolecular-weight polymers were obtained in good activities, as well as new polymer structures and much better functional group tolerance in solvent or monomer. The role of steric bulk in axial position can also be taken by a donor substituent held in an axial position above the metal center, as shown in Figure 4.95 Apparently, AlMe2Cl has to coordinate to this donor to prevent axial olefin coordination and to ensure high molecular weight.

6.06.3.5.2 Dimerization One of the simplest versions of olefin oligomerization is dimerization due to a high rate of chain transfer to propagation. Indeed, the discovery of butylene in Ziegler’s experiments on ethylene oligomerization using aluminum alkyls has led to the discovery of nickel as the responsible contaminant and a subsequent exploration of the effect of other transition metals with the discovery of ZN catalysts.

N

N Br

N Ni

Br

5 Figure 4 Nickel complex 5 with axial donor.95

6.06.3.5.3 Selective trimerization Selective trimerization cannot be achieved by simple optimization of chain transfer and propagation rates and its discovery in chromium systems has been a surprise (see Section 6.06.4.5). Selective ethylene trimerization has subsequently been reported for some titanium, vanadium and tantalum complexes, as well. A mechanism via metallacycles (see Section 6.06.2.1.3.4) is now well established and has been reviewed.96 The high demand and value of 1-hexene (and 1-octene) (mainly needed for copolymerization with ethylene) has led to a fast commercialization of ethylene trimerization. A strict control of the chromium oxidation state seems to be crucial to avoid unwanted ethylene polymerization. Re-oxidizing additives like hexachloroethane or trichloroacetates may be beneficial in some cases (see Section 6.06.5.7). Some selective trimerization catalysts are capable of cotrimerizing substituted olefins (a-olefins, styrene), resulting in a mixture of isomers. Indeed, the co-trimerization of 1-hexene (from ethylene trimerization) and ethylene leads to higher oligomers C10 and C14 in these cases. Thus, the most selective ethylene trimerization catalysts are those with no reactivity toward substituted olefins. On the other hand, so far only the sterically open triazacyclohexane complexes shown in eqn [2] show selective trimerization of a-olefins and styrene, as well.38 R R N 3

R

N

N R Cr Cl Cl Cl

MAO

R

R

R

Mixture of isomers

½2

6.06.3.5.4 Selective tetramerization Selective tetramerization was discovered by Sassol after minor modification of the PNP catalyst the use of increased ethylene

143

Oligo- and Polymerization of Olefins

pressure. The mechanism for this selectivity is still unclear (see Section 6.06.2.1.3.4).

6.06.4

Catalysts by Metal Center

Most metals in the periodic table have been used for polymerization catalysts in some form. Many metals are particularly suitable for a specific type of polymerization, as outlined below. Recently, catalysts with more than one metal center have attracted attention for the potential of cooperative effects, for example, combining ethylene oligomerization catalysts with ethylene/a-olefin copolymerization catalyst to obtain copolymers from ethylene alone in one catalysis step.97–99 Multi-metal catalysts have been reviewed previously.100 These are covered in Chapter 8.10.

6.06.4.1

Main Group Metals (Including Group 12)101

Main group organometallics are usually not suitable for coordination polymerization. However, in combination with transition metal catalysts, olefin polymers can grow on the main group metal via (reversible) chain transfer from the transition metal. ZnEt2 is frequently used for this reversible transfer and results in polymeryl zinc compounds which can be further derivatized. Hydrolysis gives saturated polymers. Cationic polymerization can be catalyzed by cationic aluminum complexes, for example, [Cp2Al]þ.102 Some polymerization activity has been observed with the aluminum complexes shown in eqn [3] upon activation with Lewis acids.

Al Me

6.06.4.3.2 Half-metallocenes107 Mono-cyclopentadienyl complexes of group 4 metals are generally less active catalysts than metallocenes. However, the

B(C6F5)3

N N

orientation of additional ligands in the ‘wedge’ of the complex (Scheme 25). The energy of the metal orbitals in the wedge is such that the central position b is the least favorable for coordination. This has important implications for the stereo-control in a-olefin polymerization. The alkyl group in the polymerization active [Cp2MR]þ will not sit in the center, and the complex is asymmetric and becomes chiral when the two cyclopentadienyl rings are not related by a mirror plane in the wedge. Exchange between the two chiral sites a and c is called ‘epimerization’. Exchange of the counter anion or solvent molecule occupying the second position in the wedge will not affect the position of the alkyl group, as exchange is usually associative, with the third ligand entering at the central position b, as shown in Scheme 25. Thus, there will be an energy difference between coordination of the re and si side of an a-olefin. This is the basis for stereo-selectivity of metallocene catalysts in a-olefin polymerization. Even greater control was achieved by Brintzinger and coworkers by linking the two cyclopentadienyl groups (‘ansametallocenes’).105 The precisely controlled ligand environment allows a rational understanding of the stereo-regularity of a-olefins, as discussed in Section 6.06.3.1.2. With the metallocene 6, shown in Scheme 25, nearly 1 million kg polypropylene per mol of Zr per hour with >99% isotactic pentade can be achieved.106

N

Me

Polyethylene 120 g mol-1bar-1h-1 at 40 °C

½3

6.06.4.2

c b a

Me2Si

M

Cl

Zr

Cl

Group 3 (Including Lanthanides)

Group 3 metallocene alkyl complexes are the neutral analogues of the cationic group 4 alkyl complexes assumed to be the active form in olefin polymerization. Thus, they have been studied in great detail as models for the group 4 catalysts. Important mechanistic studies have been done, especially on scandium complexes. Some cationic group 3 and lanthanide complexes can be highly active, but they are also much more air- and moisturesensitive than group 4 metallocenes.103 While most polymerization catalysts are mono-cationic (if not neutral) in the active form, there is some evidence that dicationic complexes may play a role for increased activity in lanthanide complexes.104 Lanthanide compounds (especially neodymium) have become important catalysts for butadiene polymerization.

6.06.4.3

M

Group 4

6.06.4.3.1 Metallocenes Bis(cyclopentadienyl) complexes offer great control over the position and, via cyclopentadienyl substituents, the

Metallocene

Ansa-metallocene 6

M

R

+ L⬘ M

L

R L⬘ L

-L M

R L⬘

L, L⬘ = anion, ligand, or solvent Scheme 25 Metallocene complexes and their ‘wedge’ positions.

R Si N

Ti Cl Cl

(CH2)n N R⬘

B N M Cl Cl

Si N

Ti Cl Cl

Figure 5 ‘Constrain geometry catalyst’ (CGC) and some variations.

144

Oligo- and Polymerization of Olefins

+2

MAO Cl 7

TiIV Cl Cl

TiII

TiIV

Other catalyst:

NO Cl Ti O Cl Cl

TiIV

8 Adamantyl Scheme 26 Selective ethylene trimerization by titanium catalysts, MAO ¼ methylalumoxane.

introduction of anionic donor substituents by Bercaw and coworkers108 20 years ago led to a surprising increase in activity when the ligand was applied to group 4. These ‘constraint geometry catalysts’ have led to one of the few industrial applications of non-metallocenes. The complexes have particularly good ethylene–a-olefin copolymerization behavior due to their open coordination sphere. Subsequently, many variations and applications in other metals have been explored and reviewed.109 Some typical examples are shown in Figure 5. A unique class of half-titanocenes includes those like 7 with pendant arene groups, shown in Scheme 26.34 Computational studies have shown the importance of the arene group, which donates as a hemilabile ligand between Z1, Z3, and Z6, depending on the requirements of titanium in the intermediates. Arene coordination has been confirmed in a number of crystal structures. Computational studies have predicted even more active catalysts based on zirconium and hafnium. However, experimental results have so far yielded only polymerization catalysts, probably via accessing a different Cossee-type mechanism. Extensive variations on the half-titanocene complexes have so far not resulted in substantial improvements in the selective trimerization. However, a recent variation on the phenoxyimine (FI) catalyst 8 shown in Scheme 26 leads to an excellent trimerization catalyst of high activity and 92% selectivity. This catalyst shows a second-order rate dependence on ethylene, suggesting that the metallacycle formation is rate-determining in this case.

6.06.4.3.3 Non-metallocenes101 Complexes without cyclopentadienyl, indenyl or fluorenyl ligands are called non-metallocenes. The success of the CGC catalysts has started an intense exploration of non- or postmetallocene catalysts, usually containing ligands with N, O, S, or P donor atoms. The ligands are often much easier to prepare and vary than the cyclopentadienyl ligands. Optimal ligands can exceed the performance of metallocene catalysts, but rational stereo control is much harder to achieve with these more flexible ligands. The next major success involved diamide complexes introduced by McConville110, as shown in eqn [4], that gave catalysts capable of living a-olefin polymerization. Subsequently,

many other diamides with various additional donor groups were explored, eventually leading to b-diketiminates and other N-donor ligands. R N

N

B(C6F5)3

Ti Me Me

R

n Living polymerization

½4 One of the most promising classes of catalysts includes alkoxide and aryloxide donor groups, for example, the salicylaldiminato complexes known as FI catalysts.111 Two of these chelate ligands form a chiral environment similar to the C2 symmetric metallocenes. Therefore, they can result in similarly good stereo control of a-olefin polymerization. A large structural diversity is accessible by straightforward synthesis, allowing the tuning to the desired polymer characteristics. For example, the molecular weight of the resulting polymer can be varied by changes of the ligand alone from 103 to 107 and to living polymerization, giving access to polymers with welldefined architectures and a wide variety of mono-disperse polymers, chain-end functionalized polymers, and block copolymers from ethylene, propylene, a-olefins, cyclic olefins, and styrene. Their synthesis and catalytic performance have been reviewed recently in a comprehensive manner.81,112 OMRP catalysts: Titanocenes and other titanium complexes used for coordination polymerization can also act as radical polymerization catalysts for styrene in the presence of a reducing reagent (e.g., Zn) via the Ti(III/IV) couple. Different from coordination polymerization, this type of radical polymerization is best performed in donating solvents, for example, dioxane.71

6.06.4.4

Group 5

Coordination polymerization113: Vanadium analogues of the classical Ziegler-type catalysts have been known since the 1950s. VCl4 was first noted by Natta for its syndiospecific low temperature polymerization of propylene probably via chain end control. They were noted for their high activity and single-site characteristics in ethylene polymerization and, more importantly, for their suitability with respect to

Oligo- and Polymerization of Olefins

copolymerization. This characteristic led to the major industrial application for vanadium catalysts for the copolymerization of ethylene, propylene and diene (synthetic EPDM rubbers, world production about one million tons per year) and also for ethylene with cyclic olefins (COC). One characteristic of vanadium is the accessibility of many oxidation states, and most pre-catalyst complexes will be reduced by the aluminum co-catalysts to often uncertain oxidation states. During catalysis, further reduction is a common cause of catalyst deactivation, resulting in short lifetimes of the active catalyst. This problem was overcome by the introduction of re-oxidants as ‘rejuvenators’ or ‘promoters’. Typical promoters are simple organics with high chlorine content, for example, trichloroacetates. Continuous addition of promoters and aluminum alkyls retains high activity even at temperatures above 100  C. The promoters are likely to oxidize inactive V(II) complexes back to active V(III) forms. A V(acac)3/Et2AlCl system is capable of sustaining syndiotactic living propylene polymerization below
65  C, which can switch to anionic block copolymerization of methyl methacrylate on warming to room temperature. The coordination chemistry and catalytic polymerization using a variety of ligands has recently been reviewed in detail, including some application of imido vanadium complexes in ROMP catalysis.113 Bis(iminopyridine)vanadium trichloride has recently been described as an OMRP catalyst for vinyl acetate.114 Generally, the heavier metals in group 5 (niobium and tantalum) are less reactive in olefin polymerization than vanadium. This has been ascribed to the stronger metal–carbon bonds. However, living ethylene polymerization has been achieved with the zirconocene analogues Cp*(diene)MCl2 (Cp* ¼ Z5-C5Me5, M ¼ Nb,Ta).115 Nevertheless, some complexes have been found active in coordination polymerization or ROMP and were reviewed recently.116 Tantalum has long been known to form metallacycles from olefins. In 2001, Sen and co-workers reported that TaCl5 with alkylating/reducing agents like AlR3, ZnR2 or SnR4 can trimerize ethylene with high selectivity but modest activity. Mashima and co-workers117 found that a slightly more active system can be obtained with bis(trimethylsilyl)cyclohexadienes as a purely organic and only reducing (but not alkylating) reagent. This suggests that reduction (probably to Ta(III)) is required, but no alkylation.34 This matches the activation of chromium in the Phillips catalyst.

6.06.4.5

Group 6

The heterogeneous Phillips catalyst based on chromium oxide is one of the most important catalysts for ethylene polymerization. Despite nearly 60 years of intensive research, the nature of the active sites is still unclear. The experimental

R⬘ N Cl S Cr S R Cl R Cl

N P

Ar

Ar P Ar Cr O Cl Cl Cl

145

evidence collected on the catalyst has been reviewed many times.35,118 Another heterogeneous catalyst (also known as Union Carbide catalyst) is based on chromocene Cp2Cr on silica, in which chromium is likely attached as CpCr units. These achieve high activity and the molecular weight can conveniently be tailored by the amount of hydrogen added to ethylene (very different to the Phillips catalyst). The polymer properties are also quite different from the Phillips catalyst products, so that the mechanism is probably different in the two systems. A large number of homogeneous model systems have been reported for both the Phillips and chromocene-derived catalysts. Theopold has been able to isolate mono-cyclopentadienyl chromium alkyl cations that are active in ethylene polymerization as good models for the heterogeneous chromocene catalysts.119–121 Mono-cyclopentadienyl chromium complexes with additional donor substituents on the Cp show high ethylene polymerization activity.122,123 Among the large number of nonCp chromium complexes, some are capable of producing polymer with similar end group and branching characteristics to the Phillips catalysts. Details can be found in more comprehensive reviews.101 Selective oligomerization34: Chromium complexes are currently the most successful catalysts for the selective trimerization and tetramerization of ethylene. There is good evidence that they occur via metallacycles and through a Cr(I/III) redox couple, as outlined in Section 6.06.2.1.3.4. Since the first description 20 years ago of highly selective catalysts based on a system of 2,5dimethylpyrrole, aluminum alkyls and a chromium(III), interest in selective trimerization has been increasing to more than a hundred publications per year. In addition, the first industrial trimerization processes based on chromium have been running for a few years now. Most publications are still using a mixture of ligand, co-catalyst and a chromium source without the isolation of well-defined complexes. However, a growing number of crystallographically characterized pre-catalyst complexes are known. In a few cases, ‘self-activating’ complexes that do not require any additional co-catalyst have been described.124–126 Some important complexes are shown in Figure 6. They show a complex interplay of various species containing the ligand, aluminum and chromium in often low oxidation states. A clear picture of the activation process and actually active complex has not yet emerged. It appears that Cr(I) species are required for selective trimerization and Cr(II) species lead to polymerization. However, change of oxidation state is facile under catalytic conditions and often depends on subtle changes in the system. In most cases, the active catalyst is accessed from Cr(III) or Cr(II) complexes. However, active PNP chromium complexes have also been obtained via oxidation from Cr(0) and CO ‘digestion’ with AlEt3, as shown in Scheme 27.

R N R N Cr Cl Cl Cl

R N

N Cr I

Al Cl N Al

Figure 6 Selection of pre-catalysts active in selective ethylene trimerization and a ‘self-activating’ complex (right).

146

Oligo- and Polymerization of Olefins

R

Ar N P Ar Ar P CO Ar Cr OC CO CO

R

+ AlEt3 +

Ar N P Ar Ar P CO Ar - Ag Cr OC CO CO A = Al(OC4F9)4

No activity with AlEt3

Ag+ A-

1-Hexene + 1-Octene

A-

and others

Scheme 27 Oxidative activation of Cr(0) complexes.

In some cases, a beneficial effect of added organic chlorides (‘halide effect’) similar to the effect of ‘promoter’ in vanadium catalysts has been observed. The effect may be caused by a reoxidizing effect of the additives or some stabilization via weak interactions with C–Cl bonds.127 In summary, chromium complexes exhibit a fine balance between catalysts active for polymerization and catalysts active for selective trimerization. The oxidation state seems to play an important role, with Cr(II) and Cr(III) apparently involved in polymerization and Cr(I) in trimerization. However, most precatalysts are Cr(III) complexes. As a general rule, cationic Cr (III) complexes with an anionic ligand (e.g., Cp) and one alkyl group tend to give polymerization catalysts, while cationic Cr (III) complexes with neutral ligands and two alkyl groups lead to selective trimerization. Radical polymerization69,71: The couple Cr(II/III) can be used for radical polymerization. The redox potential can be shifted to a suitable range by the addition of polyamine ligands to Cr(acac)2 or the use of b-diketiminato complexes, and reasonable activities were found for the polymerization of vinyl acetate and similar polar olefins. The heavier group 6 metals are little used as coordination polymerization catalysts. Molybdenum complexes of the oxidation states between III and VI have been used for OMRP or ATRP of styrene. Molybdenum complexes have been among the first welldefined metathesis catalysts suitable for a broad range of applications, including ROMP, developed by Schrock and coworkers.128 Chiral catalysts could be used to produce chiral polymer via ROMP.129

6.06.4.6

Group 7

Despite the enormous success in finding olefin polymerization catalysts for most transition metals, very little progress has been made with group 7 metals. There has been one report of controlled radical polymerization of styrene, vinyl acetate and methacrylate using irradiated [Mn2(CO)10] via a reactive [Mn(CO)5] radical. The system [ReO2I(PPh3)2]/Al(OiPr)3 has been used for radical polymerization of styrene.69

6.06.4.7

Group 8101

Iron complexes analogous to the diimine nickel catalysts show only limited activity. Alkyl abstraction from an isolated dialkyl complex with B(C6F5)3 leads to aryl transfer from boron and an inactive complex.130 However, tridentate ligands lead to catalysts of high activity for ethylene polymerization.

R⬘

R N

N Fe X X

R⬘

R

R⬘⬘

N

O R

R 8

L

Ni

N R

9

Figure 7 Typical iron and nickel pre-catalysts.

Propylene and 1-butene are dimerized to mainly internal linear hexenes and octenes, respectively.131 A particularly successful ligand has been tridentate bis (imino)pyridines 8, shown in Figure 7, and structural variations thereof in the form of penta-coordinated FeCl2 precatalysts have also proven successful.111 Imino substituents can block access to axial sites, as with the diimine nickel catalysts, and – more importantly – prevent the formation of bis-chelate complexes. They produce strictly linear oligomers to high molecular polymers of ethylene. Besides b-hydrogen transfer, chain transfer to aluminum is frequently observed leading to saturated polymer end groups on work-up and a bi-modal molecular weight distribution. The oxidation and spin state of the active form of iron in these catalysts is still controversial. EPR and Mo¨ssbauer studies show that iron is oxidized to Fe(III) on addition of MAO and Fe(III) pre-catalysts produce the same polymer suggesting that the same active species is formed. In addition a DFT study suggests that polymerization via [LFe(III)R]2þ is feasible.132 However, Chiric and co-workers133 were able to isolate the [LFe(II)R]þ complex shown in Scheme 28, which is active for linear ethylene polymerization. This contradiction might have been resolved by invoking redox-non-innocent bis(imino)pyridine ligands. However, a careful study of the [LFeR]þ complex by a variety of spectroscopic and computational methods showed the ligand to be redox-innocent in this case. However, one- and two-electron reduction led to ligand rather than iron reduction.134 A recent computational study based on a quartet spin state of [LFe(III)R]2þ is able to predict the degree of oligomerization and isomers of the 1-butene dimerization based on the Cossee–Arlman mechanism.135 Ruthenium catalysts for the ethylene polymerization are very rare. A computational study on the best polymerization catalyst shows that all typical polymerization pathways considered are high in energy and that multinuclear species may be required to achieve polymerization.136 Iron and also ruthenium and osmium have been used extensively for controlled radical polymerization of styrene, methacrylate and similar olefins via the (II/III) redox couple. A system based on RuCl2(PPh3)2, described in 1995, has been

Oligo- and Polymerization of Olefins

N N

N

Fe Si

B(C6F5)3

Si

147

N N

PE

N

Fe Si

[MeB(C6F5)3] Si

Scheme 28 Isolated and active [LFeR]þ cation.

one of the first ATRP catalysts.73 The polymerization can be controlled by a variety of ligands with nitrogen, phosphorus, and other donor groups. Many of the complexes used for coordination polymerization can also give good radical polymerization catalysts.69,71 The most important application of ruthenium in olefin polymerization is as metathesis catalyst for ROMP and ADMET (see Section 6.06.2.2).

6.06.4.8

Group 9

Cobalt complexes analogous to iron complexes 8 (shown in Figure 7), especially bis(imino)pyridine CoCl2 complexes, lead to catalysts of high activity. Generally, their activity is about one order of magnitude lower than the activity of analogous iron complexes. As for iron, oxidation states from I to III have been implicated for the active species. The complexes LCoCl2 can be reduced to cobalt(I) and corresponding complexes [LCoCl], [LCoR] and, after alkyl abstraction with B(C6F5)3, [LCo(N2)]þ have been isolated. The cobalt(I) complexes do not seem to be active toward ethylene polymerization. However, oxidation of [LCoR] with ferrocenium salts leads to [LCoR]þ salts, which can be isolated and used as single component catalysts for ethylene polymerization.137 A variety of cobalt complexes has been used for controlled radical polymerization via the Co(II/III) couple. Cobalt complexes are the oldest and most mature OMRP catalysts. These systems are also reported under the name ‘cobalt-mediated radical polymerization’ (CMRP). The process relies on a dormant organocobalt(III) species generating the radical and a square planar Co(II) complex in the active form. The catalyst was discovered during the study of model complexes for vitamin B12, which utilizes a similar homolytic Co–C bond cleavage for its activity. The CMRP process has been applied to produce a variety of block-copolymers of polar olefins and allowed the controlled synthesis of various polymer architectures, for example, star copolymers, that tolerate a variety of functional groups in the monomer. CMRP polymers have been used in many specialty applications.71 In addition, the redox active cobaltocene can be used as ATRP catalyst for methyl methacrylate. In this case, RX reacts with Cp2Co to give a cobalt(I) complex that acts as ATPR catalyst via a Co(I/II) cycle. Some Rhodium complexes have also been used for radical polymerization.69

industrial process to access C10 to C20 olefins (see Section 6.06.3.5.1). Polymerization4,101: One type of simple catalyst precursors are planar (a-diimine)NiX2 complexes (X typically halides) where the diimine can be easily varied at the imine substituent or the backbone. Brookhart and co-workers discovered that steric bulk in the axial positions can be easily introduced via the N-substituents. This leads to a dramatic change in reactivity toward polymerization upon activation, typically with MAO. The diimine ligands can be easily varied by different substituents and is an ideal system for catalyst optimization.138 Analogous palladium complexes show a similar reactivity but allow NMR observation of the process due to slower rates. Under high ethylene pressure (10 bar or more) and reduced temperature (5  C), ethylene polymerization using palladium catalysts can become living. NMR analyses and variation of the precursors have resulted in a detailed understanding of the mechanism, as outlined in Scheme 20. Electron-donating substituents in the ligand were found to lead to increased catalyst lifetime and higher molecular weight.139 As discussed in Section 6.06.3.2.1, chain walking can lead to highly branched polymers. Molecular weights of PE of over 1 million can be reached and can be controlled by the addition of hydrogen, silanes, or CBr4. The catalyst can be supported on various inorganic oxides including silica. The lower oxophilicity of nickel and especially of palladium leads to a higher tolerance of polar functional groups. Ethylene can be successfully copolymerized with methyl acrylate. Other functionalities are also tolerated such as ethers, sulfones, fluorides, and even epoxides. Functionalities are also tolerated as solvents so that polymerization can also be done in alcohols. Nitrogen-based functionalities (nitriles, amides, and amines) in the monomer or solvent generally inhibit polymerization. A large variety of complexes with anionic ligands similar to the SHOP catalyst have been studied. When sufficient steric bulk in the axial position is introduced, these can become polymerization catalysts. A particularly successful and easily variable system involves salicylaldiminato complexes 9, as shown in Figure 7.33 Most of these nickel complexes are highly selective for ethylene and will only dimerize a-olefins. Nickel and palladium complexes have also been used as radical polymerization catalysts.69

6.06.4.10 6.06.4.9

Group 10

Nickel is the classical metal for olefin oligomerization and oligomerization with the neutral SHOP catalysts is a major

Group 11

Copper has become the most important metal for controlled radical polymerization via ATRP using the Cu(I/II) cycle (see Chapter 6.04). ATRP catalysts are dominated by complexes of

148

Oligo- and Polymerization of Olefins

nitrogen donor ligands. The redox potential of copper complexes can be finely tuned by the choice of ligand, and copper has a suitable halogen affinity. Ligand effects have been shown to be very important, and the catalyst becomes more active when the ligand stabilizes the Cu(II) state, with activities ranging over many orders of magnitude depending on the ligand. The results have been explored extensively in a recent review.69,72,140

6.06.5

Co-Catalysts, Activators, and Promoters5

The active species in most polymerization catalysts are cationic metal–alkyl complexes. While such active catalysts have been isolated in some cases, most applications will use a combination of a more stable and readily available pre-catalyst complex with co-catalysts. The transformation into the active cationic alkyl complex can be divided into two steps, as shown in Scheme 29: first, the alkylation of the complex (often methylation) and, second, the generation of the cationic complex by the abstraction of one anionic ligand(see Section 6.06.2.1.2). In many cases, the two steps are combined by a single cocatalyst, while in others the metal–alkyl is isolated as the precatalyst and the co-catalyst only needs to generate the cation. As metal–alkyls can be readily converted into cations by a variety of methods, dialkyl complexes are a convenient precursor, where one alkyl group is sacrificed to generate the cation. This simple activation scheme is often complicated by the formation of alkyl or X-bridged aggregates between the cationic and neutral complex and by the coordination of the generated anion or other products of the anion abstraction process, yielding the actual active cationic complex in equilibrium with these adducts. Therefore, the catalyst activity greatly depends on the choice of co-catalyst. As most active polymerization catalysts are sensitive to air and moisture, the alkylating co-catalyst can also have the dual role of a scavenger. Therefore, these co-catalysts, usually aluminum alkyl compounds, are added in large excess.

on computational studies. Computationally speaking, the most stable cage structure contains 12 Al atoms. However, open nanotubular structures with AlMe3 caps were found to be more stable, with an energy minimum at a formula of Me24Al16O12 in good agreement with experimental composition and molecular weight.143 Upon anion abstraction, the size of the oligomer increases to 150–200 Al atoms. In addition, MAO contains variable amounts of AlMe3 and its concentration can strongly influence the catalytic behavior. This AlMe3 leads to additional species present in the activated catalysts containing coordinated (m-Me)2AlMe2, which seem to slow down catalytic activity in many cases. Thus, MAO freed of AlMe3 is often a more effective activator. The metal m-polymeryl cations occurring during polymerization are much less likely to form analogous AlMe3 adducts. A large part of the AlMe3 in MAO solutions can be removed under vacuum and redissolution of the remaining white solids. Alternatively, MAO solutions can be treated with 2,6-di-tert-butyl cresol (BHT), which reacts selectively with AlMe3.

6.06.5.2

Other aluminum alkyl compounds AlR3 (R ¼ Me, Et, iBu,. . .) or AlRnX3
n (X mostly Cl) have been used as co-catalysts from the beginnings of metal catalyzed polymerization. They are excellent alkylating and scavenging reagents and are capable of abstracting anions to some extent. In particular, triisobutylaluminum (TIBA) is added to other co-catalysts as a cheap scavenger, often with activity enhancing properties. One of the reasons for the enhanced activity may be the increased steric bulk of any isobutyl group containing anions, leading to increased ion separation. Indeed, TIBA modified MAO (MMAO) has become a popular co-catalyst over the recent years with extended shelf life, often in addition to better activity. However, TIBA can lead to increased catalyst deactivation if the catalyst is susceptible to metal reduction.

6.06.5.3 6.06.5.1

Methylalumoxane141

All of these functions (alkylation, anion abstraction, and scavenging) are fulfilled by the most widely used co-catalyst, methyl alumoxane (MAO). In fact, the serendipitous discovery of MAO as partially hydrolyzed AlMe3 in the mid-1970s led to the breakthrough for homogeneous polymerization catalysis.77 Despite 30 years of important and extensive research, the species present in MAO are known only approximately: [Me1.4–1.5AlO0.75–0.80]16.142 Thus, solutions of MAO appear to contain oligomers of around 16 Al atoms, with (MeAlO) and some AlMe3 units containing one unsaturated Lewis acidic site of the type –OAlMe2– and one of the type –Al (O)2Me– each for every 100 Al. No crystal structure of MAO itself has been reported so far, and structural information relies

LnMX2

Co-catalyst

LnMRX

Boranes

Boranes with sufficiently bulky and electron-withdrawing substituents can be used as Lewis acids for the abstraction of an anionic ligand to give an ion pair. Usually, the resting state of this ion pair still has a strong contact to the cation or even a bridging group. However, separation into a free cation can be a low-energy process that is accessible during catalysis. The most important borane used in polymerization catalysis is B(C6F5)3, which is a readily available stable solid. Methyl abstraction leads to a [MeB(C6F5)3]
anion that is sufficiently weakly coordinating to dissociate partially into ions and allow polymerization. Benzyl groups can be abstracted in a similar way. A number of crystal structures of the resulting ion pair show coordination to the cation via the methyl group, as shown in Scheme 30. B(C6F5)3

X− abstracting

Alkylating

Other Aluminum Alkyl Compounds

[LnMR ] A− +

Co-catalyst

Scheme 29 Activation by alkylation and anion abstraction.

LnMMe2

Me LnM [LnMMe+] [MeB(C6F5)3]H H H C B(C F ) 6 53

Scheme 30 Activation by boranes.

Oligo- and Polymerization of Olefins

F F B(C6F5)3

Cp2Zr

F

B(C6F5)3 F

F

n

B(C6F5)2

R Cp2Zr

Cp2Zr

Ar N Ni N Ar

R n

B(C6F5)3 Ar = 2,6-PriC6H3

149

(C6F5)3B

Ar N Ni N Ar

Polyethylene

Scheme 31 Zwitter-ionic complexes by borane addition.

[HD+]ALnMMe2

Me + LnM AD

Me + LnM A- + D

(typically Me) of the cationic Lewis acid leads to a neutral non-coordinating triphenylethane and the selected weakly coordinating anion.

Scheme 32 Activation with Brønsted acids.

6.06.5.6 The use of B(C6F5)3 together with AlR3 can lead to C6F5 group exchange between boron and aluminum, often with deactivation of both co-catalysts. A large number of differently substituted boranes and analogous alanes have been explored, generally showing a range of activity. Boranes are also used as Brønsted acidic co-catalyst in combination with weakly acidic HX, giving weakly coordinating anions [X(B(C6F5)3)n]
(n often 2 with X ¼ CN, NH2, F,. . .). Boranes or other Lewis acids can also lead to active zwitterionic complexes. The Lewis acid either can react with a suitable precursor ligand for the metal–alkyl group (e.g., a butadiene complex)144 or can react with a donor site in the ancillary ligand.145 An example for each case is shown in Scheme 31.

6.06.5.4

Brønsted Acids with Weakly Coordinating Anions

Catalyst precursors with protolyzable anionic ligands (usually alkyls) can react with Brønsted acids to give active cations. This method allows the introduction of well-defined anions. Weakly coordinating anions supporting high solubility in noncoordinating solvents are usually chosen. Typical and readily available anions are [B(C6F5)4]
or [Al(OC(CF3)3)4]
. The proton acids of these anions are only available as Lewis base adducts [HDþ]. But the Lewis bases D can be chosen to be weakly coordinating enough to allow polymerization. [PhNMe2H]þ or [H(OEt2)2]þ are typical examples of such acids (Scheme 32). The generation of the acid by the combination of borane with HX discussed in preceding section allows Brønsted acid activation without any donor base. As the alkane formation in the protolysis step is irreversible, this type of activation is very efficient.

Oxidizing reagents are sometimes used to remove an oxidation sensitive ligand in order to generate a free coordination site, for example, metal–alkyl bonds can be cleaved with silver(I) salt. Another typical oxidants is ferrocenium. A variety of weakly coordinating anions are available for these oxidants, including [B(C6F5)4]
or [Al(OC(CF3)3)4]
. These oxidizing reagents can also be used to reach the active cationic metal complex by oxidation from a neutral lowervalent complex.137

6.06.5.7

Trityl Salts of Weakly Coordinating Anions

An activator combining the two preceding sections is a cationic Lewis acid analogue of the borane with a weakly coordinating anion. A commonly used co-catalyst of this type are trityl salt, for example, [CPh3]þ [B(C6F5)4]
. Anion abstraction

Oxidizing Additives (Halide Effects)

Some catalysts require additives to control the oxidation state of the metal. Oxidants have to be able to react with overreduced metal without reacting with the reducing organometallic reagents in the system, for example, organo aluminum compounds. Typical oxidizing additives are highly chlorinated organics, for example, hexachloroethane, hexachlorocyclopentadiene, or trichloroacetate. They are most commonly used for vanadium catalysts (see Section 6.06.4.4), but are also used for some chromium systems.127

6.06.5.8

Nucleating Additives

An industrially important class of additives controls the nucleation of the polymer particles. Progress in this area has been reviewed recently146 and will not be discussed further here.

6.06.6 6.06.6.1

6.06.5.5

Oxidizing Salts of Weakly Coordinating Anions

Monomers for Olefin Polymerization Ethylene

Polyethylene (PE) is by far the most common olefin polymer. Despite the simple monomer structure, a huge variety of different polymers can be produced, resulting in range of properties and applications. Apart from varying chain length (up to molecular weights of several millions for ultrahigh-molecular-weight

150

Oligo- and Polymerization of Olefins

But3P

N

Ti

+ Me

LnTi

But

Me But

B(C6F5)4 +

But Me

But +

LnTi

But

But

But

LnTi

LnTi

But

H But

But +

But

But

Scheme 33 s-Bond metathesis in catalysis with bulky olefins.

P N

P

N

L

N

M N

N

P

N

L

N

M N

M

P L

M

P L

N M

Chain walking

N

Chain walking

N

n

L P

n

M N

L

Scheme 34 Polymerization of cyclic olefins coupled with chain walking.

polyethylene (UHMWPE)), the structure can vary from highly linear (HDPE) through linear polymer with some branching (LLDPE) to highly branched LDPE with branches on branches. Apart from the high temperature/pressure ICI polymerization to LDPE, ethylene is usually polymerized by coordination polymerization. However, polyethylene can also be obtained from metathesis polymerization followed by hydrogenation. Recently, polyethylene has also been prepared in cyclic form via REMP (see Section 6.06.2.2).

6.06.6.2

a-Olefins

The most common a-olefin polymer is polypropylene. Higher a-olefins can also be homo-polymerized, but they are more commonly used for copolymerization with ethylene. They are commonly polymerized by coordination polymerization. Bulky a-olefins may have difficulties with metal–alkyl bonds for steric reasons and react by s-bond metathesis instead. This is illustrated by the example of neohexene dimerization shown in Scheme 33.147

due to the release of ring strain. Polycyclic olefins with even higher ring strain, such as norbornene, are also excellent monomers for ROMP, as well as for coordination polymerization. Due to its ready availability, many studies have been done on cyclopentene. The symmetric nature simplifies the NMR analysis of the polymer product. Diimine catalysts of both nickel and palladium produce a crystalline polymer that is quite different from metalloceneproduced isotactic polymer. Chain walking allows the formation of more stable trans-1,3-polymer as shown in Scheme 34.149 As monomer insertion occurs at the sterically least hindered site accessible by chain walking, methylene or ethylene bridged 1,3-cyclopentane polymers are formed selectively. Polymers with cycloalkanes in the backbone can also be obtained from the polymerization of non-conjugated a,odienes. Catalysts capable of reacting with a-olefins can also react with dienes. After the insertion of the first double bond, an intra-molecular cyclization by insertion of the second double bond occurs, as shown in Scheme 35.

6.06.6.4 6.06.6.3

Cyclic Olefins and Non-conjugated Dienes148

The predominant use of cyclic olefins for polymerization is in ROMP, as discussed in Section 6.06.2.2.1. However, there are also catalysts available for coordination polymerization. Small ring cyclic olefins (cyclopentene or smaller) have a higher propensity to polymerize relative to other disubstituted olefins

Higher Substituted Olefins

Disubstituted olefins such as trans-2-butene are still a difficult substrate for coordination polymerization. The bulky secondary metal–alkyl usually prevents the coordination and subsequent insertion of additional bulky monomer. Thus, early transition metal catalysts are usually inactive toward these olefins. However, chain walking in late transition metal

Oligo- and Polymerization of Olefins

LnM

LnM LnM–P X

151

P X

X

P

X

n

Scheme 35 Cyclo-polymerization of a,o-dienes.

But

R2 R1

Me3Si O

S

O M

S

X X

Cl Cl

Ti

Ph N

Cl Cl

allyl

Cl

Nd

O

O 10 R1

R2

Ti

O S

THF

N

M = Ti, Zr, Hf X = Cl, OPri, benzyl

But

11

12

>100,000 kg PS/(molTih)

Isospecific

THF

13

>99% Syndiotactic Syndiospecific

Figure 8 Typical titanium pre-catalysts for styrene polymerization.

catalysts has opened a new route to polymerization, as the metal can chain-walk to a terminal carbon before insertion.150 Highly substituted olefins are susceptible to cationic polymerization, for example, polyisobutylene (see Section 6.06.2.3).

6.06.6.5

Styrene151–154

Polystyrene is one of the most important industrial polymers, with several million tons produced per year. Most of it is produced via free radical polymerization, with no need for metal catalysts. However, styrene polymerization can be catalyzed by most mechanisms outlined above and the mechanism of metal-catalyzed styrene polymerization may vary from case to case. Coordination polymerization, in particular, can produce highly stereospecific polystyrene, for example, iso- and syndiotactic polystyrene, with very different properties from the common atactic polystyrene. Isotactic polystyrene was obtained from Ziegler-type catalysts as early as 195,5 and heterogeneous catalysts have been improved to achieve >99% isotacticity and molecular weights in the millions. However, all heterogeneous systems also produce some atactic polymer that has to be removed by solvent extraction. Isotactic polystyrene from homogeneous coordination polymerization has been a great challenge, but some systems based on ansa-bis(indenyl) complexes of lanthanides and group 4 metals produce highly enriched isotactic polystyrene. A particularly successful non-metallocene system has been the dianionic [OSSO] ligand 10, shown in Figure 8, which can be resolved into enantiomerically pure catalysts by the use of a 1,2-cyclohexene instead of ethylene bridge.153 However, isotactic polystyrene has not gained much commercial interest, mainly due to its slow crystallization rate. Syndiotactic polystyrene has a much higher crystallization rate and melting points above 260  C have been achieved. It can be produced using a variety of transition metal complexes via coordination polymerization. Monocyclopentadienyl

complexes of titanium either with halides or other anionic ligands (phenoxides 11, amides 12, imides) give highly syndiotactic polystyrene when activated with MAO. Generally, titanium is more active than analogous zirconium complexes (this is different from a-olefin polymerization). Typical complexes are shown in Figure 8. The mechanism of the syndiotactic styrene polymerization was investigated in great detail, especially for halftitanocenes. The active species seems to be [CpTiIIIR]þ obtained after alkylation, anion abstraction and reduction from the titanium(IV) precursors. In some cases, natural light is required to achieve the reduction. 13C NMR studies have shown that the styrene insertion is strictly secondary (or 2-1) opposite to the dominant orientation of the a-olefin insertion.155 Analysis of the polymer microstructure shows that the insertion of styrene is chain-end controlled in these systems, and the incoming styrene avoids steric repulsion from the last inserted phenyl group, such that a syndiotactic sequence results. Recently discovered ansa-lanthanidocenes with Ewen-type fluorenyl and Cp groups 13 have been shown by mechanistic and computational studies to be strictly syndiotactic, due to the interaction of the styrene with the fluorenyl ligand.155 More control over anionic polymerization can be achieved by retarded anionic polymerization.68

6.06.6.6

Butadienes156

Butadiene can be polymerized in three different ways resulting in cis, trans, and vinyl-polymers and each can be syndio-, iso-or a-tactic. The properties of polybutadiene are different, depending on the proportion of these isomers. They are mainly used as synthetic rubbers, as the remaining vinyl groups can be used for cross-linking. Butadiene (and other dienes like isoprene) can be polymerized by radical or anionic polymerization, with little control over the polymer structure.

152

Oligo- and Polymerization of Olefins

As for styrene, coordination polymerization with metal catalysts has an important influence on the type of polymer produced. High cis-polybutadiene is characterized by a high proportion of cis (typically over 92%) and a small proportion of vinyl (<4%). It is manufactured using ZN catalysts. Depending on the metal used, the properties vary slightly. Iron and especially cobalt complexes of N and P ligands can give catalysts for high trans-1,4-polybutadiene. Neodymium gives the most linear structure (and therefore higher mechanical strength) and a higher percentage of 98% cis. Other less frequently used catalysts include nickel and titanium. Low cis-polybutadiene is prepared using alkyl lithium catalysts in anionic polymerization. The catalyst produces a ‘low cis’-polybutadiene which typically contains 36% cis, 54% trans, and 10% vinyl. Because of its high liquid-glass transition, low cispolybutadiene is not used in tire manufacturing, but it can be used advantageously as an additive in plastics, due to its low contents of gels. High vinyl polybutadiene (>80% 1,2 polymerization) has been achieved with (diphosphane)Cr(II)Cl2 complexes activated with MAO. This material is produced with an alkyl lithium catalyst. A type of polybutadiene with 90% vinyl has the properties of an elastomeric thermoplastic: elastic at room temperature, but a fluid at high temperatures, which makes it possible to process it using injection molding. Polybutadiene can be produced with more than 90% trans using catalysts similar to those of high cis: neodymium, lanthanum, and nickel. This material is a plastic crystal (i.e., not an elastomer) that melts at about 80  C. It was formerly used for the outer layer of golf balls. Today it is only used industrially, but companies like Ube are investigating other possible applications. The use of metallocene catalysts to polymerize butadiene seems to give a higher degree of control both in the distribution of molecular mass and the proportion of cis, trans, and vinyl.

6.06.6.7

CO (for copolymers)157–159

While CO is not an olefin, it can be copolymerized with olefins in a remarkably selective, perfectly alternating fashion and the most important palladium catalysts for this copolymerization are similar to palladium homo-oligo-polymerization catalysts.

6.06.6.8

Other Functionalized Monomers159

Olefins bearing functional groups can also be polymerized. In many cases, the functional group sufficiently activates the olefins, such that that metals are required to achieve polymerization, for example, for vinyl chloride, acrylates, or acrylonitrile.

6.06.7

Heterogeneous Polymerization Catalysts

Industrial polymerization of olefins is usually based on heterogeneous, or at least heterogenized, catalysts, due to the

great simplifications of the large scale processes. The main industrial olefin polymerization catalysts are heterogeneous ZN and Phillips catalysts. Heterogeneous catalysts tend to have many different reactive sites and observation of the reactive species by spectroscopy is much more difficult than for homogeneous catalysts. Nevertheless, great control over the polymerization has been achieved via the catalysts preparation and the use of additives. The fundamental processes on the heterogeneous catalyst sites will be similar to those of welldefined homogeneous systems and can often be studied in greater detail in the latter. Therefore, this chapter will focus on homogeneous catalysts. Heterogeneous catalysts are covered in detail in Volume 7. Only the main features of heterogeneous polymerization catalysts will be covered here with respect tothe ZN and Phillips catalysts.

6.06.7.1

ZN Catalysts

The development of ZN catalysts from their early discovery in the 1950s is usually been described in terms of four ‘generations.’ The first generation, the original ZN catalyst, is based on solid TiCl3 containing about 30 mol% AlCl3 from the reduction of TiCl4. Activation with Et2AlCl leads to a good polymerization catalyst. Some base additives were able to improve the activity by selectively coordinating catalyst poisons, such as EtAlCl2, present in the co-catalyst. The study of the isospecific propylene polymerization has led Cossee and Arlman to develop the basic mechanistic principles of the coordination polymerization.8 Second-generation ZN catalysts were developed by removing most of the AlCl3 and converting the catalyst to the more active d-TiCl3 phase (5–20 kg PP per gram of catalyst). The third-generation ZN catalysts were based on TiCl4 on MgCl2 support and included internal donors (e.g., ethyl benzoate) to affect the crystallite formation and preferred surface form. Together with AlR3 and external donors as additives, the activity was approximately doubled. Reduction of the electron donors led to fast catalyst deactivation. Introduction of diesters like phthalates or diethers as electron donors led to the currently used fourth-generation catalysts, with activities in the order of 100 kg polypropylene (PP) per gram of catalyst.

6.06.7.2

Phillips Catalysts

The chromium-based Phillips catalyst is used to produce nearly 50% of the world’s high-density polyethylene. Despite over 50 years of intensive research in industry and academia, the precise structure and mechanism of the active site remains unclear. The Phillips catalyst has been reviewed extensively78 and only the main points will be covered here. One characteristic property of polyethylene from the Phillips catalyst is the occurrence of mainly methyl and butyl branching. A recent study shows that the frequency of methyl branches correlates with the amount of dinuclear chromium sites. Thus, propylene may be formed at multinuclear sites and incorporated into the polymer giving methyl branches. The occurrence of butyl branches increases with the yield of the polymerization, as would be expected from 1-hexene formed via selective trimerization during the catalysis.160

Oligo- and Polymerization of Olefins

6.06.8

Conclusion

Metal-catalyzed olefin polymerization has made great progress since its initial discovery by Ziegler 60 years ago. Most metals have been shown to catalyze polymerization by a range of mechanisms, and most olefinic monomers can be polymerized under suitable conditions. The degree of oligo- to polymerization can be controlled, from selective di-, tri-, and tetramerization all the way to ultrahigh-molecular-weight polymers. A range of polymer structure can be obtained by using different catalysts, and different monomers can be copolymerized with some degree of control, most notably in blockcopolymerization using living catalysts or the precisely alternating CO-olefin polymerization. At the same time, the activities of catalysts have become so high that diffusion control becomes limiting in some cases. Does this mean that catalytic olefin polymerization is a mature field, as discussed by some reviews?161,162 I think a great deal more could be achieved by comparing current olefin polymerization with what nature could achieve for the polymerization of amino acids: precise chain lengths of copolymers of 20 monomers, bearing a broad range of functional groups in a precise sequence and in high yield, with control over the polymer folding and mechanisms to de-polymerize and recycle the monomers.

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